Removed references to old callback mechanism

src/include/etherboot.h

 ***************************************************************************/
 /* main.c */
 struct Elf_Bhdr;
-extern int in_call(in_call_data_t *data, uint32_t opcode, va_list params);
 extern void console_init(void); 
 extern int main();
 extern int loadkernel P((const char *fname));
 /* floppy.c */
 extern int bootdisk P((int dev,int part));
 
-/* pxe_callbacks.c */
-extern int pxe_in_call ( in_call_data_t *in_call_data, va_list params );
-
 /***************************************************************************
 External variables
 ***************************************************************************/