Remove $global mistakenly left in test

This was accidentally left in when this change was [submitted][1] to
main. This was already addressed in the backport commit but this
addresses it for main.

[1]: 57a9e25

Author: skipkayhil

activerecord/test/cases/migration/compatibility_test.rb

@@ -783,8 +783,6 @@ def migrate(x)
       end
     }.new(nil, 0)
 
-    $global = true
-
     change_migration = Class.new(migration_class) {
       def migrate(x)
         change_column :more_testings, :published_at, :datetime