Drop ROMCC code and header guards

Change-Id: I730f80afd8aad250f26534435aec24bea75a849c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37334
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/include/halt.h

@@ -17,14 +17,10 @@
 #ifndef __HALT_H__
 #define __HALT_H__
 
-#ifdef __ROMCC__
-#include <lib/halt.c>
-#else
 /**
  * halt the system reliably
  */
 void __noreturn halt(void);
-#endif /* __ROMCC__ */
 
 /* Power off the system. */
 void poweroff(void);