Clean up labels css

What does this MR do?

This MR cleans up labels css used in the project labels, group labels and admin labels pages. Unnecessary css is removed and utility classes are used instead in some instances in preparation for feature work in #241990 (closed).

Screenshots

	before	after
project labels
project labels mobile
group labels
group labels mobile
admin labels
admin labels mobile
group milestones
project milestones

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team