utils: use g_usleep() instead of my_usleep()

Now that I've found this nice function in the GLib docs, we can
finally remove our custom sleep function.  Still all those callers of
g_usleep() have to be migrated one day to use events, instead of
regular polling.

src/decoder_api.c

@@ -24,7 +24,6 @@
 #include "audio.h"
 #include "song.h"
 
-#include "utils.h"
 #include "normalize.h"
 #include "pipe.h"
 
@@ -129,7 +128,7 @@ size_t decoder_read(struct decoder *decoder,
 
 		/* sleep for a fraction of a second! */
 		/* XXX don't sleep, wait for an event instead */
-		my_usleep(10000);
+		g_usleep(10000);
 	}
 }