Suppressing MergeableSelector warnings in scss-lint from EE-only files

To get ready for merging gitlab-org/gitlab-ce!12810, MergeableSelector warnings in EE-only files and selectors should be fixed.

It is not a tough work at all since there are only 9 warnings over 4 files existed only in EE (See https://gitlab.com/gitlab-org/gitlab-ee/issues/3873#note_45123099).

Links

• meta: #3873 (closed)
• CE: gitlab-org/gitlab-ce#23485