arch: remove stage_exit()

It's no longer used. Remove it.

Change-Id: Id6f4084ab9d671e94f0eee76bf36fad9a174ef14
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13678
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/arch/arm/include/arch/stages.h

@@ -19,6 +19,5 @@
 extern void main(void);
 
 void stage_entry(void);
-void stage_exit(void *);
 
 #endif