Remove last traces of discard_all_inferiors

Remove last traces of discard_all_inferiors

The multi-target patch should have removed all traces of
discard_all_inferiors, but somehow one use stayed behind along with
the definition of the function.

discard_all_inferiors is bad now because it blindly exits inferiors of
all target connections.  It's best to remove it.

gdb/ChangeLog:
2020-01-12  Pedro Alves  <palves@redhat.com>

	* bsd-kvm.c (bsd_kvm_target::close): Call exit_inferior_silent
	directly for the current inferior instead of
	discard_all_inferiors.
	(discard_all_inferiors): Delete.