Remove unnecessary spec usage_activity_by_stage spec

What does this MR do?

• This spec takes 35 seconds (search here) https://gitlab-org.gitlab.io/rspec_profiling_stats/
• But it's already tested in the core usage data spec https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/usage_data_spec.rb#L15
• After removing it we get rid of the double call to uncached_data and get rid of the slowest test

Issue #241260 (closed)

Before

$ bundle exec rspec -fd ee/spec/lib/ee/gitlab/usage_data_spec.rb

Finished in 2 minutes 20 seconds (files took 23.27 seconds to load)
27 examples, 0 failures

After removing the spec

$ bundle exec rspec -fd ee/spec/lib/ee/gitlab/usage_data_spec.rb

Finished in 2 minutes 4.4 seconds (files took 23.51 seconds to load)
26 examples, 0 failures

Queries saved: 3157

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