Tweak labeler config (#27502)

Alternative to https://github.com/go-gitea/gitea/pull/27439. Removes a
few spammy labels, and disables `sync-labels` which make it never remove
labels (which is default behaviour).

.github/workflows/pull-labeler.yml

@@ -18,4 +18,3 @@ jobs:
       - uses: actions/labeler@v4
         with:
           dot: true
-          sync-labels: true