EE Port: Fix style-lint errors and warnings for `app/assets/stylesheets/framework/forms.scss`

EE-port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28129

EE specific changes

Change	Effect	Notes	Before	After
Remove .form-label style definition	Project Settings -> General -> Merge Request Approvals	Replaced with common.scss utility classes

Conformity

• Changelog entry
• Code review guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Tested in all supported browsers