Remove unneeded comments about feature support on the adapters

These comments will likely be outdated with time and doesn't include
any information that can't be found in the adapters

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -146,28 +146,24 @@ def adapter_name
         'Abstract'
       end
 
-      # Does this adapter support migrations? Backend specific, as the
-      # abstract adapter always returns +false+.
+      # Does this adapter support migrations?
       def supports_migrations?
         false
       end
 
       # Can this adapter determine the primary key for tables not attached
-      # to an Active Record class, such as join tables? Backend specific, as
-      # the abstract adapter always returns +false+.
+      # to an Active Record class, such as join tables?
       def supports_primary_key?
         false
       end
 
-      # Does this adapter support using DISTINCT within COUNT? This is +true+
-      # for all adapters except sqlite.
+      # Does this adapter support using DISTINCT within COUNT?
       def supports_count_distinct?
         true
       end
 
       # Does this adapter support DDL rollbacks in transactions? That is, would
-      # CREATE TABLE or ALTER TABLE get rolled back by a transaction? PostgreSQL,
-      # SQL Server, and others support this. MySQL and others do not.
+      # CREATE TABLE or ALTER TABLE get rolled back by a transaction?
       def supports_ddl_transactions?
         false
       end
@@ -176,8 +172,7 @@ def supports_bulk_alter?
         false
       end
 
-      # Does this adapter support savepoints? PostgreSQL and MySQL do,
-      # SQLite < 3.6.8 does not.
+      # Does this adapter support savepoints?
       def supports_savepoints?
         false
       end
@@ -185,7 +180,6 @@ def supports_savepoints?
       # Should primary key values be selected from their corresponding
       # sequence before the insert statement? If true, next_sequence_value
       # is called before each insert to set the record's primary key.
-      # This is false for all adapters but Firebird.
       def prefetch_primary_key?(table_name = nil)
         false
       end
@@ -200,8 +194,7 @@ def supports_partial_index?
         false
       end
 
-      # Does this adapter support explain? As of this writing sqlite3,
-      # mysql2, and postgresql are the only ones that do.
+      # Does this adapter support explain?
       def supports_explain?
         false
       end
@@ -211,14 +204,13 @@ def supports_transaction_isolation?
         false
       end
 
-      # Does this adapter support database extensions? As of this writing only
-      # postgresql does.
+      # Does this adapter support database extensions?
       def supports_extensions?
         false
       end
 
       # Does this adapter support creating indexes in the same statement as
-      # creating the table? As of this writing, only mysql does.
+      # creating the table?
       def supports_indexes_in_create?
         false
       end
@@ -231,14 +223,12 @@ def disable_extension(name)
       def enable_extension(name)
       end
 
-      # A list of extensions, to be filled in by adapters that support them. At
-      # the moment only postgresql does.
+      # A list of extensions, to be filled in by adapters that support them.
       def extensions
         []
       end
 
       # A list of index algorithms, to be filled by adapters that support them.
-      # MySQL and PostgreSQL have support for them right now.
       def index_algorithms
         {}
       end
@@ -299,7 +289,6 @@ def clear_cache!
       end
 
       # Returns true if its required to reload the connection between requests for development mode.
-      # This is not the case for Ruby/MySQL and it's not necessary for any adapters except SQLite.
       def requires_reloading?
         false
       end