Use @gitlab/ui color in member badges

What does this MR do?

Uses @gitlab/ui color ($purple-600 #694cc0) in member badges instead of arbitrary color #6d49cb, which was introduced in !28257 (merged) and !28243 (merged). Unused CSS rule introduced in !21285 (merged) for another purpose is also removed.

Part of #212259 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.