Do not need `- 2` margin for `allowed_index_name_length` in SQLite3

Actually SQLite3 doesn't have a limitation for identifier name length,
so do not need `- 2` margin internally to perform temporary rename
operations.

activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb

@@ -91,6 +91,10 @@ def create_table_definition(*args, **options)
             SQLite3::TableDefinition.new(self, *args, **options)
           end
 
+          def validate_index_length!(table_name, new_name, internal = false)
+            super unless internal
+          end
+
           def new_column_from_field(table_name, field)
             default = \
               case field["dflt_value"]

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -179,13 +179,6 @@ def supports_index_sort_order?
         true
       end
 
-      # Returns 62. SQLite supports index names up to 64
-      # characters. The rest is used by Rails internally to perform
-      # temporary rename operations
-      def allowed_index_name_length
-        index_name_length - 2
-      end
-
       def native_database_types #:nodoc:
         NATIVE_DATABASE_TYPES
       end