Re-enable docs generation (#24453)

Hopefully Firebase is all done compacting now, so we're going to try and build docs again.

dev/bots/docs.sh

@@ -119,9 +119,6 @@ fi
 # Ensure google webmaster tools can verify our site.
 cp "$FLUTTER_ROOT/dev/docs/google2ed1af765c529f57.html" "$FLUTTER_ROOT/dev/docs/doc"
 
-# TODO(gspencer): Re-enable this when Firebase hosting is fixed.
-exit 0
-
 # Upload new API docs when running on Cirrus
 if [[ -n "$CIRRUS_CI" && -z "$CIRRUS_PR" ]]; then
   echo "This is not a pull request; considering whether to upload docs... (branch=$CIRRUS_BRANCH)"