Remove nptl/sockperf.c.

This patch removes nptl/sockperf.c, an unused, x86-specific program
with a hardcoded path in /tmp.  If someone finds some of this code in
future for adding a proper benchmark, that does not of course rule out
adding it back in that form, but for now I think it's best to
eliminate this code with the hardcoded /tmp path.

Tested for x86_64.

	* nptl/sockperf.c: Remove file.