[pp-1] Move or delete olivero_drupal_alter_alter

For more context, this was originally added by #591794: Allow themes to alter forms and page → in 2009.

This might sound confusing as olivero isn't that old. But it was just renamed, I guess a few times. In 2009 it was originally added as garland_drupal_alter_alter.

Originally, this was a test to make sure that themes can implement every alter hook, with a made up hook name only used in that test. This was changed, theme alters only work for specific, explicitly called alter hooks, and the test was extend in #2228093: Modernize theme initialization → and lost its original purpose already kind of then.

IMHO, we have test coverage of test themes implementing actual alter hooks and we could just drop that hook and that part of the test, but I'm also fine wit just moving the hook to test_theme or something.