main: Remove the block height from the coinbase, since it is now in the header.
main: Remove the block height from the coinbase, since it is now in the header.

Summary: Small change to remove BIP-0034 soft-fork-required height in coinbase

Reviewers: johann, ben

Reviewed By: ben

Differential Revision: https://phabricator.bitgo.com/D4228

BitGo/prova,BitGo/prova