remove deprecated `ActiveRecord::Base#connection` method.

activerecord/CHANGELOG.md

+*   Remove deprecated `ActiveRecord::Base#connection` method.
+    Make sure to access it via the class.
+
+    *Yves Senn*
+
 *   Remove deprecation warning for `auto_explain_threshold_in_seconds`.
 
     *Yves Senn*

activerecord/lib/active_record/core.rb

@@ -337,14 +337,6 @@ def readonly!
       @readonly = true
     end
 
-    # Returns the connection currently associated with the class. This can
-    # also be used to "borrow" the connection to do database work that isn't
-    # easily done without going straight to SQL.
-    def connection
-      ActiveSupport::Deprecation.warn("#connection is deprecated in favour of accessing it via the class")
-      self.class.connection
-    end
-
     def connection_handler
       self.class.connection_handler
     end

activerecord/test/cases/fixtures_test.rb

@@ -493,10 +493,6 @@ class CustomConnectionFixturesTest < ActiveRecord::TestCase
   fixtures :courses
   self.use_transactional_fixtures = false
 
-  def test_connection_instance_method_deprecation
-    assert_deprecated { courses(:ruby).connection }
-  end
-
   def test_leaky_destroy
     assert_nothing_raised { courses(:ruby) }
     courses(:ruby).destroy