Remove CODE16 as a global define; most files define it themselves

anyway, and I want to be able to use it as a compile-time option for
the i386 decompressor.

src/include/etherboot.h

 #define	TAGGED_IMAGE		/* choose at least one */
 #endif
 
-#undef CODE16
-#if defined(PCBIOS)
-#define	CODE16
-#endif
-
 #define K_ESC		'\033'
 #define K_EOF		'\04'  /* Ctrl-D */
 #define K_INTR		'\03'  /* Ctrl-C */