[POWERPC] Remove stale comment from head_64.S

This is now inaccurate because we may not have entered prom_init() and
r3 is overwritten immediately anyway.

Signed-off-by: Sonny Rao <sonny@burdell.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/head_64.S

@@ -1734,10 +1734,6 @@ _STATIC(__boot_from_prom)
 	/* We never return */
 	trap
 
-/*
- * At this point, r3 contains the physical address we are running at,
- * returned by prom_init()
- */
 _STATIC(__after_prom_start)
 
 /*