Typo Fix In decoderawtransaction Help: s/txid/hex/

Help text appears to have been copy/pasted from getrawtransaction,
so it erroneously asked for a txid where rawtransaction hex should appear.

Remove lines which were copy/pasted from getrawtransaction but which
aren't displayed by decoderawtransaction.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 5cc0133 80c521e
Github-Pull: #4106