The "exploitable vulnerability" may lead to a misunderstanding that
missed hardening issues are considered vulnerabilities, just that
they're not exploitable.  This is not true, since while hardening bugs
may be security-relevant, the absence of hardening does not make a
program any more vulnerable to exploits than without.

Drop the "exploitable" word to make it clear that missed hardening is
not considered a vulnerability.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>

ChangeLog:

	* SECURITY.txt: Drop "exploitable" in the hardening section.