Don't attempt to remove a non-existing qwk-pack semfile

Reduce occurrences of subsequent errors (unsure of root-cause):
!ERROR 2 (No such file or directory) in main.cpp line 2778 (event_thread) removing "/sbbs/data/pack0341.now"

This does seem to be the only location that deleted pack*.now files, so weird that they'd disappear sometime after calling glob(), but I do understand that its theoretically possible.