Remove some cleanups from probe.c

Remove some cleanups from probe.c

This removes some cleanups from parse_probes by using std::string; and
removes some unnecessary cleanups from elsewhere in probe.c.

ChangeLog
2017-10-16  Tom Tromey  <tom@tromey.com>

	* probe.c (parse_probes): Use std::string.
	(info_probes_for_ops, enable_probes_command)
	(disable_probes_command): Remove cleanups.