Drop target method 'fbsd_make_corefile_notes'

Drop target method 'fbsd_make_corefile_notes'

Now that all users of the target method 'fbsd_make_corefile_notes'
have been converted to the version in fbsd-tdep.c, the old method is
removed.

gdb/ChangeLog:

	* fbsd-nat.c (find_signalled_thread, find_stop_signal)
	(fbsd_collect_regset_section_cb, fbsd_make_corefile_notes):
	Remove.
	* fbsd-nat.h (fbsd_make_corefile_notes): Remove prototype.