CI: Remove irrelevant branch from master GitHub Actions config (#1292)

CI: Remove irrelevant branch from master GitHub Actions config (#1292)

The original GitHub Actions configuration (see commit 2ed8cc4)
incorrectly assumed that GitHub will use that configuration (stored in
the master branch) for testing _all_ repository branches. In reality,
GitHub uses configuration from the being-tested branch. Thus, we do not
need to (and, hence, should not) list any branches in the X branch
configuration, with the exception of the X branch itself and the "auto"
branch where all PRs (including all X-targeting PRs) are staged for the
final "as if merged" testing.