Update socketcan_linux.go

[rainervonuns]

Clean up code: removed unused function, which caused a failure during cross build for 32bit architecture.
Bits word width varies by arch (int64 on amd64/arm64, int32 on 386/arm).

Alternative could be:

func fdSetAdd(set *syscall.FdSet, fd int) {
	const wordBits = 8 * int(unsafe.Sizeof(set.Bits[0]))
	set.Bits[fd/wordBits] |= 1 << uint(fd%wordBits)
}

Since the function is never called --> remove