Remove _POSIX_REGEX_VERSION

There is no _POSIX_REGEX_VERSION, so don't check for it.
_REGEX_VERSION has been removed as well[1], so only keep the -1 return
for backward compatibility.  I found this when trying to make the
getconf environment variables typo-proof.

	* sysdeps/posix/sysconf.c (__sysconf): Return -1 for
	_SC_REGEX_VERSION.

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/sysconf.html