Revert "Issue #3158374 by SKAUGHT: Show Drupal Icon depending on which drupal major version it is"

https://www.drupal.org/project/admin_toolbar/issues/3550270

Remove unused Tools icon for Drupal 8:

• Reverted commit 46ca3837 which introduced this feature: git revert 46ca3837f2e118983c670065b6d8947d9b2f93ec and fixed conflicts.
• Renamed CSS file tools.css to admin_toolbar_tools.css.
• Refactored admin_toolbar_tools.css CSS code and added inline comments.
• Added a cache rebuild on module update.