Check for "Uninstall" button reducing friction

What does this MR do?

Make await_installed match both possible button labels Installed and Uninstall. Also, remove await_uninstallable to reduce friction if we more cluster applications are uninstallable.

This was mentioned by @DylanGriffith in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30251#note_187773883:

I think this is fine for now since this is broken but I would say that it would be nice if instead the implementation detail was not leaked to the caller. await_installed should be clever enough to realise that Uninstall button also indicates that the application is installed. I don't think this should hold up this MR but we should consider that refactoring.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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