Remove obsolete code in decode.go

[fxamacker]

Issue To Be Solved

Several type assertions in decode.go are not needed because they immediately follow a function that returns the desired type.

And decodeBig has obsolete code for v1 compatibility.

Suggestion

Remove type assertions that immediately follow a function call that returns the desired type.

Remove the obsolete code in decodeBig. The code was made obsolete by the recent decision to remove backward compatible decoding from the upcoming v4 decoder.

Context

Removal of these code blocks do not change v4 storage format.

Preliminary benchmark comparisons show speed gains of 3-4% decoding a 167 byte CBOR LinkValue.

Indirectly updates #738 because this was discovered while reviewing code to prepare for finalization of v4 storage format.

[turbolent]

@fxamacker Is it OK to close this issue given that PR #788 was merged?