Allow deleting non-existing build requests
Allow deleting non-existing build requests

Rather than have an extra call to query if a build request exists
before deleting it (which is racey to begin with unless locks are
used), just try deleting it and hide the exception if the request
doesn't exist.

Change-Id: If6ac40c7246ea2486cb76ba6f01ae3fdca2d9f5e

Tesora/tesora-nodepool,Tesora/tesora-nodepool