ci(doxygen): remove stale branch guard blocking the tag-triggered job

The job trigger was narrowed to push:tags a while back, but the
job-level 'if: github.ref == refs/heads/master' was left in place.
A tag push never has github.ref == refs/heads/master, so the two
conditions are mutually exclusive: the job trigger fires only on
tag pushes, while the guard only allows master-branch refs, meaning
the job has been silently skipped on every run since the trigger
was narrowed.

This is also the likely source of the recent Git LFS bandwidth/
storage overage: while the trigger was still push-to-master (pre-
narrowing), this job ran on nearly every commit and committed a
new QElectroTech.qch (LFS-tracked) each time, via the auto-generated
update-qch PR -- accumulating one LFS object version per run."

.github/workflows/auto-doxygen.yml

@@ -11,7 +11,6 @@ jobs:
     permissions:
        contents: write    
     runs-on: ubuntu-latest
-    if: github.ref == 'refs/heads/master'
     steps:
     - uses: actions/checkout@v4
       with: