Remove test report count endpoint

Ref: #229693 (closed)

What does this MR do?

Following up !37629 (merged), where we remove the feature flag for our ~performance improvement of our JUnit feature.

This MR removes an endpoint no longer being used to generate the number of total tests being ran for a pipeline.

Why are we doing this?

Before the introduction of BuildReportResult we were parsing/loading all test reports in memory and then caching the total_count displayed below.

Today we don't need to do this anymore as we are reading the total_count directly from the database as you can see on this example pipeline.

Screenshots

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