Fix retry_failed_e2e_rspec_examples for single parallel run

What does this MR do and why?

I think there is an edge case where a single test can be executed in one parallel process, and when a test fails it retries as expected. If the test passes, we try to delete tmp/rspec-*-retried-false*.xml but in the case of only one parallel process there is no tmp/rspec-*-retried-false*.xml files as the only one was moved.

This causes a job failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/11545880307#L438

This MR adds the -f flag to rm so the command does not fail.