nb/intel/i440bx: Drop mainboard_enable_serial()

All boards using this northbridge now enable serial in bootblock,
so this is no longer needed.

Change-Id: I6baf2de81870dbba2a7f1abb3f1fdd6716d64511
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41048
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/northbridge/intel/i440bx/raminit.h

@@ -9,7 +9,6 @@
 void enable_spd(void);
 void disable_spd(void);
 void sdram_initialize(int s3resume);
-void mainboard_enable_serial(void);
 
 /* Debug */
 #if CONFIG(DEBUG_RAM_SETUP)