Remove str-truncated-60 class from common.scss

What does this MR do?

Removes str-truncated-60 class from app/assets/stylesheets/framework/common.scss. The class was formerly used, but completely removed in !58212 (merged) and !58373 (merged) so now we can safely remove it.

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because it is too small.
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] 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

Closes #326634 (closed)