Don't delete pack####.now upon FTP-logoff

When a user logins to the FTP server concurrently, this creates a
race condition with an/the other FTP session that may be creating/
downloading a QWK packet. On Vertrauen, this results in the
occasional error removing the file since it was removed unexpectedly:
!ERROR 2 (No such file or directory) in main.cpp line 2747 (event_thread) removing "/sbbs/data/pack1111.now" access=0