lib: drop return value from igt_drop_caches

No one actually cares, everyone expects it to just work.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/gem_persistent_relocs.c

@@ -294,7 +294,7 @@ static void do_forked_test(int fd, unsigned flags)
 		igt_fork_helper(&thrasher) {
 			while (1) {
 				usleep(1000);
-				do_or_die(igt_drop_caches_set(val));
+				igt_drop_caches_set(val);
 			}
 		}
 	}