Remove legacy transaction decoding.
Remove legacy transaction decoding.

This is no longer needed since the tests that used it were updated.  It
really should have never been in the production code anyways.  Code only
needed by tests should be in the associated test package.

jrick/btcd,decred/dcrd,jrick/dcrd,jrick/btcd,jrick/dcrd,jrick/btcd,decred/dcrd