doc: thumbnailer: remove _POSIX_CLOCK_SELECTION

Since e56d967334, there is no
clock_gettime() call amd there is no need for this POSIX feature to be
enabled.

Refs #28519

doc/libvlc/thumbnailer.c

@@ -28,13 +28,6 @@
 #include <pthread.h>
 #include <errno.h>
 #include <time.h>
-
-#if !defined (_POSIX_CLOCK_SELECTION)
-#  define _POSIX_CLOCK_SELECTION (-1)
-#endif
-
-static_assert (_POSIX_CLOCK_SELECTION >= 0, "Clock selection unavailable!");
-
 #include <vlc/vlc.h>
 
 /* position at which the snapshot is taken */