operations: Always use quarantine in UserCreateTag
The use of manual object quarantine directories for UserCreateTag has been introduced via eca4b3c2 (operations: Use object quarantine directories in UserCreateTags, 2021-07-08). With this change in place, the new tag object is created in a manually created object quarantine directory, which can then be used to both speed up access checks and to discard tags which aren't allowed by our access checks.
Remove the feature flag. The code has been tested since August 2nd without any issues and should thus be safe to enable.
Changelog: performance
Closes #3706 (closed)