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/utils.h

@@ -28,8 +28,6 @@
 	} while (0)
 #endif /* !assert_static */
 
-void my_usleep(long usec);
-
 char *parsePath(char *path);
 
 int set_nonblocking(int fd);