SYMPTOM:
When deleting an attribute, following error message appears:

Cannot delete '_REMOVE_Store' because other objects depend on it. Do you want to search the dependent objects?

Enabling the 'Display managed objects' option in the Search Editor results in one more managed System Hierarchy as dependent object:

CAUSE:
This is caused by logical metadata inconsistency. In order to track down the cause of the inconsistency, customers encountering this issue should make note of Schema-related changes, Object Migrations, or error messages prior to experiencing the problem. Change Journal, if it has been enabled, can help in preserving this information.
ACTION:
In this particular case, invalid references exist in all three dependent objects. Contact Strategy Technical Support for further assistance.