[CI] Don't override the default Junit file name

Summary:
Since teamcity is using all the files starting with the default name
(with a wildcard) and this not targeting human reading, there is no
point in having different names unless we have multiple runs.

Test Plan:
Check the CI build output for this diff and ensure the junit files are
still being interpreted by the CI.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4769