Fix issue reported by Va7aqd: when sub-boards were cut to the clipboard

(or deleted) or a group of sub-boards was deleted, and any of the deleted/cut
sub-boards belonged to a configured QWKnet hub, that QWKnet sub-board would
end-up pointing to the next sub-board in the configuration (possibly in another
message group, possibly not part of the QWKnet in question). Now, if a
QWKnet sub is deleted/cut, remove it (correctly) from any QWKnet hubs using
a common (new) function: remove_sub(). The bug was: the remaining sub-board
pointers were moved *before* the remove-sub pointer was compared against the
list of QWKnet hub subs (and NULLed), so while the effort *was* made to address
this problem, it didn't actually work in practice. Thanks for the bug report.