refactor(oauth): remove superfluous join in QR login tests

[Johennes]

As discussed in #5840 (comment), the joins are not needed because two of the awaited tasks were already spawned.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.60%. Comparing base (10ff5d0) to head (2ec6e61).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5850      +/-   ##
==========================================
- Coverage   88.60%   88.60%   -0.01%     
==========================================
  Files         361      361              
  Lines      102294   102287       -7     
  Branches   102294   102287       -7     
==========================================
- Hits        90637    90628       -9     
- Misses       7438     7439       +1     
- Partials     4219     4220       +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5850 will not alter performance

_{Comparing Johennes:johannes/no-join (2ec6e61) with main (10ff5d0)}

Summary

✅ 50 untouched