Remove __attribute__ ((unused)), use -Wno-unused-parameter instead

The __attribute__ ((unused)) is useful sometimes to declare variables as
intended to be unused, e.g. in callback functions.  However, they tend
to clutter up the code making hard to read.  We therefore remove all
uses of it and instead set -Wno-unused-parameter.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/timer.c

@@ -7,7 +7,7 @@
 int result = -1;
 struct timeval start;
 
-static void cb(uev_t *w, void *UNUSED(arg), int events)
+static void cb(uev_t *w, void *arg, int events)
 {
 	struct timeval now;