scripts: set_assignee: don't skip assignment on too many labels

The set assignee script currently exits if there's too many labels
associated with a PR. Change that to just skip the label assignment, but
continue on to assign a maintainer based on the most relevant area.

Tested with:

  ./scripts/set_assignees.py -v --dry-run -P 52716

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>