Remove Unused Helpers from `tab_helpers.rb`

What does this MR do?

In a separate branch I was creating a custom tab_helper and while trying to find an example of usage, I noticed a couple had been defined but were no longer being used. I imagine they had be replaced in their templates but not removed from the helper file.

I went ahead and deleted them since it was a bit confusing for me trying to see how to use a custom tab helper. This way any others won't get tripped up and we can clean up the codebase a bit.

Screenshots

Usage of project_tab_class:

Usage of profile_tab_class:

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

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

cc: @mkozono