Remove unused <threads.h> header

This was introduced before for some threading implementation to have
definitions for 'tss_t' and 'tss_set()' and similar, but these were
later removed, while the header was still kept.

This header doesn't exist on older systems with older glibc and is the
reason for the only patch we need to have in Chromium to make PipeWire
to build.

spa/plugins/support/loop.c

@@ -9,7 +9,6 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <pthread.h>
-#include <threads.h>
 #include <stdatomic.h>
 
 #include <spa/support/loop.h>