Call the new point behavior `:point`, not `:rails_5_1_point`

Since the attributes API is new in Rails 5, we don't actually need to keep
the behavior of `attribute :point`, as it's not a breaking change.

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -816,9 +816,8 @@ def construct_coder(row, coder_class)
         ActiveRecord::Type.register(:json, OID::Json, adapter: :postgresql)
         ActiveRecord::Type.register(:jsonb, OID::Jsonb, adapter: :postgresql)
         ActiveRecord::Type.register(:money, OID::Money, adapter: :postgresql)
-        ActiveRecord::Type.register(:point, OID::Point, adapter: :postgresql)
+        ActiveRecord::Type.register(:point, OID::Rails51Point, adapter: :postgresql)
         ActiveRecord::Type.register(:legacy_point, OID::Point, adapter: :postgresql)
-        ActiveRecord::Type.register(:rails_5_1_point, OID::Rails51Point, adapter: :postgresql)
         ActiveRecord::Type.register(:uuid, OID::Uuid, adapter: :postgresql)
         ActiveRecord::Type.register(:vector, OID::Vector, adapter: :postgresql)
         ActiveRecord::Type.register(:xml, OID::Xml, adapter: :postgresql)

activerecord/lib/active_record/model_schema.rb

@@ -385,7 +385,7 @@ def warn_if_deprecated_type(column)
 
             If you'd like the new behavior today, you can add this line:
 
-              attribute :#{column.name}, :rails_5_1_point#{array_arguments}
+              attribute :#{column.name}, :point#{array_arguments}
           WARNING
         end
       end

activerecord/test/cases/adapters/postgresql/geometric_test.rb

@@ -7,10 +7,10 @@ class PostgresqlPointTest < ActiveRecord::PostgreSQLTestCase
   include SchemaDumpingHelper
 
   class PostgresqlPoint < ActiveRecord::Base
-    attribute :x, :rails_5_1_point
-    attribute :y, :rails_5_1_point
-    attribute :z, :rails_5_1_point
-    attribute :array_of_points, :rails_5_1_point, array: true
+    attribute :x, :point
+    attribute :y, :point
+    attribute :z, :point
+    attribute :array_of_points, :point, array: true
     attribute :legacy_x, :legacy_point
     attribute :legacy_y, :legacy_point
     attribute :legacy_z, :legacy_point