virtualbox: build BIOS from C instead of alternative sources

VirtualBox ships with "alternative BIOS sources" for its virtual BIOS.
These are generated by first compiling the BIOS C sources with the
Open Watcom toolchain, disassembling the output and checking in the
disassembly into the VirtualBox repo.

The result means that the BIOS C code cannot be patched, because it's
not compiled from the C sources, if Open Watcom is not there.

As Open Watcom is now available in nixpkgs, we can just ignore the
alternative BIOS sources and compile it from C directly.