devel/py-cbor: No longer cater to GCC 4.2

Remove code catering to GCC 4.2 (as system compiler) which is not
necessary any longer since USES=compiler and USE_GCC never will pull
that in anymore.

This reverts much of commits 029e8b89818d7d3a576a5a172fefc93d42799ad6
and a00547aa72ab3b19fa4ac7e625957268b5bde020 from 3+ years ago.

Approved by: sunpoet (maintainer)