Remove unused fixtures in *ConnectionTest

These fixtures were added in 9a4e183fe9f81365f7815c391512b3d46622d8c8 to
test the behavior of `connection.truncate`, but then the relevant tests
were extracted to `AdapterTestWithoutTransaction` in
fdac932707fb16b7d074b1d0bc7c255157d72675.

activerecord/test/cases/adapters/abstract_mysql_adapter/connection_test.rb

@@ -6,8 +6,6 @@
 class ConnectionTest < ActiveRecord::AbstractMysqlTestCase
   include ConnectionHelper
 
-  fixtures :comments
-
   def setup
     super
     @subscriber = SQLSubscriber.new

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

@@ -10,8 +10,6 @@ class PostgresqlConnectionTest < ActiveRecord::PostgreSQLTestCase
     class NonExistentTable < ActiveRecord::Base
     end
 
-    fixtures :comments
-
     def setup
       super
       @subscriber = SQLSubscriber.new