Remove workaround for bug in Linux kernel 2.6.8 or thereabouts.

Remove workaround for bug in Linux kernel 2.6.8 or thereabouts.
The workaround isn't strictly needed for POSIX conformance, and
it's too much of a pain to configure and maintain.  We'll ask
people to fix their kernels instead.
* modules/xnanosleep (Depends-on): Remove gethrxtime.
* lib/xnanosleep.c: Don't include gethrxtime.h or xtime.h.
(NANOSLEEP_BUG_WORKAROUND): Remove.
(xnanosleep): Remove the workaround.
* m4/xnanosleep.m4 (gl_XNANOSLEEP): Remove configuration attempting
to detect nanosleep bug.