Fix issue with QWKnet hub sub-board mappings when inserting new sub-boards
The sub_t.subnum's that were being updated as message areas were being saved to msgs.ini could not be used as an index into the scfg_t.sub array at this point (the subnum would be the new index position when the msgs.ini was re-read/loaded) . This was not an issue in v3.19 because we just saved the subnum (to msgs.cnf) and in v3.20, we save the sub's internal code (to msgs.ini) and were using the newly updated sub_t.subnum to find the corresponding sub_t for that internal code. Since the subnum is not used now during the save process, no need to update it here (this reverses part of the commit 11e529d4 from 5 years ago). This fixes issue #502 - thanks to the irc.synchro.netizens that reported it!