prepare_for_detach: don't release scoped_restore at the end

After detaching from a process, the inf->detaching flag is
inadvertently left set to true.  If you afterwards reuse the same
inferior to start a new process, GDB will mishave...

The problem is that prepare_for_detach discards the scoped_restore at
the end, while the intention is for the flag to be set only for the
duration of prepare_for_detach.

This was already a bug in the original commit that added
prepare_for_detach, commit 24291992 ("PR gdb/11321"), by yours
truly.  Back then, we still used cleanups, and the function called
discard_cleanups instead of do_cleanups, by mistake.

gdb/ChangeLog:

	* infrun.c (prepare_for_detach): Don't release scoped_restore
	before returning.