scheduler/job.c: unload job before freeing job history in cupsdDeleteJob()

scheduler/job.c: unload job before freeing job history in cupsdDeleteJob()

With "PreserveJobHistory Off", LogLevel not set to debug (or debug2),
and "LogDebugHistory 200" (the default), cupsdDeleteJob() frees the
job history and then unloads the job.  However, unload_job() calls
cupsdLogJob() which re-creates the job history and puts "Unloading..."
into it because level (debug) is greater than LogLevel (warn) and
LogDebugHistory is set to 200 messages by default.  Unused (and
unreachable) job history is left behind, resulting in a memory leak.