Debugger: Lift restriction one-shot breakpoints deletion

This was/is a workaround to not lose the extra information carefully
crafted one-shot-breakpoints might carry (e.g. conditions). However,
it gets into the way when simple one-shot breakpoints are used
automatically ("Jump to line", or native-mixed debugging),
so actually deleting the bp in the ui if it's gone on the debugger
side seems the lesser evil nowadays.

Change-Id: If00dc4e8f27881236c05b17109b57d7c53d424ab
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>