Remove dual autoloader feature from global Drush installs

[greg-1-anderson]

Do not allow Drush to combine autoload files from a non-site-local Drupal; only bootstrap Drupal sites with a shared vendor.

Something I did on the train for Drush 12.

[greg-1-anderson]

This feature was untested. Removing it did not break anything else.

[greg-1-anderson]

Ready for review, but should only be merged into a 12.x branch.

[weitzman]

Reminder that there is a bit of Drupal autoloader code in https://github.com/drush-ops/drush/blob/d6d05512b32ea4dd4983e40b0e0a6e30236050f2/src/Commands/core/CacheCommands.php#L220

[greg-1-anderson]

Continued in #5521 (12.x version)