Update libm-test.inc comment about NaN signs.

libm-test.inc has a comment about signs of NaNs not being tested.
This is out of date since:

commit ce665817
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Sat Nov 16 12:48:35 2013 +0000

    Test signs of NaNs in libm-test.inc where appropriate.

This patch removes the inaccurate statement.

	* math/libm-test.inc: Update comment not to refer to signs of NaNs
	not being tested.