platforms: remove TODO

It turns out Mac OS X v10.11.6 (El Capitan) is still covered by security
updates at this time, so not end-of-lifed, so we might as well keep
these legacy things.

src/platforms/px4_time.h

@@ -5,7 +5,6 @@
 #include <pthread.h>
 
 #if defined(__PX4_APPLE_LEGACY)
-// TODO: check if this is required
 #define clockid_t unsigned
 #endif