Update API Branches Details test

Endpoint has been fixed as of 12.6 and now performs to expectations. Removing custom threshold and tags.

* Environment:    25k
* Version:        12.7.0-pre `f347c4bd9a4`
* Option:         60s_500rps
* Date:           2020-01-20
* Run Time:       38m 11.92s (Start: 13:17:55 UTC, End: 13:56:07 UTC)
NAME                                                     | RPS   | RPS RESULT           | RESPONSE P95 | REQUEST RESULTS | RESULT
---------------------------------------------------------|-------|----------------------|--------------|-----------------|-------
api_v4_projects_repository_branches_branch               | 500/s | 488.13/s (>160.00/s) | 55.40ms      | 100.00% (>95%)  | Passed

Note that the issue this was tagged under was technically incorrect (my mistake). That covered its sister Branches List API, which is still not fixed.