chore(#352): revert release-signed.yml codegen (belongs on dev's version)

My previous commit added the codegen step to this branch's OLD
android-signed release-signed.yml. But dev has since replaced that file
with a 5-job pipeline (#342/#346), so editing the stale copy made both
sides diverge -> PR #348 went CONFLICTING and CI stopped running.

release-signed.yml only runs on v* tags, never on this PR, so it does
not belong in this PR at all. Restored to the merge-base version so it
merges cleanly (dev's version wins). The codegen for the release
pipeline will be added to DEV's release-signed.yml in a follow-up, AFTER
this merges (build_runner isn't a dep on dev until then).

The PR-relevant codegen (build.yml, flutter_dart.yml, deploy-web.yml) +
the .g.dart untracking are unchanged and still green the PR.

For #335 / PR #348. Agent: SapphireCompass (session 8d755b5e)

.github/workflows/release-signed.yml

@@ -100,7 +100,6 @@ jobs:
           echo "Keystore decoded (${SIZE} bytes), key.properties written."
 
       - run: flutter pub get
-      - run: dart run build_runner build --delete-conflicting-outputs
 
       # Same mechanism as build.yml (#331). A signed release that shipped
       # without the Giphy key would have a dead GIF picker.