44824: release-19.1: sql: handle missing job when during mutations GC r=pbardea a=pbardea

There are cases where the schema changer tries to clean out the
mutations on a descriptor. When it cannot find the job it currently
throws an error and this does not let the transaction that cleans the
job from the descriptor commit. However, if it cannot find the job then
there is no chance it will find the job later. Currently, the schema
changer will repeatedly attempt to clean this job and always fail since
it cannot find the job.

It should consider the job as successful if it cannot find the job and
continue committing the transaction that does the cleanup for this
mutation.

Release note (bug fix): Stop repeatedly looking for non-existing jobs,
which may cause  high memory usage, when cleaning up schema changes.

Co-authored-by: Paul Bardea <[email protected]>