Remove fields from BPB that are EBPB actually

This is just a trick so i can use bpb_size in the kernel later,
omitting things that are not relevant to interfacing the FS.

inc/bpb.asm

@@ -11,8 +11,4 @@ struc bpb
 .heads:		resw 1 ; WORD number of sides
 .sectoroffset:	resd 1 ; DWORD partition offset
 .longtotal:	resd 1 ; DWORD large total sectors
-.drivenum:	resb 1 ; BYTE drive number
-.flags:		resb 1
-.signature:	resb 1 ; BYTE 28h
-.serial:	resb 4 ; DWORD serial
 endstruc