boot/bios_ia32: Remove the unused flat_buffer field.

Suggested by korli in #11019. We pass this struct directly to the
BIOS, so pack it while we are here.

Checked against GRUB (it also does not have this field.)

Change-Id: I9a2708c44ff48fd702175053f2889443d6f7a003
Reviewed-on: https://review.haiku-os.org/c/1145
Reviewed-by: Adrien Destugues <[email protected]>

src/system/boot/platform/bios_ia32/devices.cpp

@@ -48,8 +48,7 @@ struct disk_address_packet {
 	uint16		number_of_blocks;
 	uint32		buffer;
 	uint64		lba;
-	uint64		flat_buffer;
-};
+} _PACKED;
 
 static const uint16 kParametersSizeVersion1 = 0x1a;
 static const uint16 kParametersSizeVersion2 = 0x1e;