MIPS: Remove superfluous check for __linux__

When building BPF code using "clang -target bpf -c", clang does not
define __linux__.

To build BPF IR decoders the include linux/lirc.h is needed which
includes linux/types.h. Currently this workaround is needed:

https://git.linuxtv.org/v4l-utils.git/commit/?id=dd3ff81f58c4e1e6f33765dc61ad33c48ae6bb07



This check might otherwise be useful to stop users from using a non-linux
compiler, but if you're doing that you are going to have a lot more
trouble anyway.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21149/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org