ci: remove dead inherited .github/workflows (pwn-request + cruft)

CI security audit finding. These are all inherited NewPipe GitHub workflows,
none functional on Forgejo — the real pipeline is .forgejo/workflows/.

Highest risk: pr-labeler.yml ran on pull_request_target with pull-requests:write
and called a mutable @main third-party action with GITHUB_TOKEN — a textbook
pwn-request on a public repo (base-repo write token, triggerable by any fork PR).
image-minimizer.yml ran arbitrary JS with issues/PR write perms. The rest
(no-response, backport-pr, ci, build-release-apk) are dead + were spamming a red
X on every push. Removing the lot drops third-party supply-chain surface to zero.

Also dropped changed-lines-count-labeler.yml (dead config for the deleted labeler).

.github/workflows/build-release-apk.yml

@@ -1,38 +0,0 @@
-name: "Build unsigned release APK on master"
-
-on:
-  workflow_dispatch:
-
-jobs:
-  release:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v6
-        with:
-          ref: 'master'
-
-      - uses: actions/setup-java@v5
-        with:
-          distribution: 'temurin'
-          java-version: '21'
-          cache: 'gradle'
-
-      - name: "Build release APK"
-        run: ./gradlew assembleRelease --stacktrace
-
-      - name: "Rename APK"
-        run: |
-          VERSION_NAME="$(jq -r ".elements[0].versionName" "app/build/outputs/apk/release/output-metadata.json")"
-          echo "Version name: $VERSION_NAME" >> "$GITHUB_STEP_SUMMARY"
-          echo '```json' >> "$GITHUB_STEP_SUMMARY"
-          cat "app/build/outputs/apk/release/output-metadata.json" >> "$GITHUB_STEP_SUMMARY"
-          echo >> "$GITHUB_STEP_SUMMARY"
-          echo '```' >> "$GITHUB_STEP_SUMMARY"
-          # assume there is only one APK in that folder
-          mv app/build/outputs/apk/release/*.apk "app/build/outputs/apk/release/NewPipe_v$VERSION_NAME.apk"
-
-      - name: "Upload APK"
-        uses: actions/upload-artifact@v7
-        with:
-          name: app
-          path: app/build/outputs/apk/release/*.apk