Don't include the padding in WirePayForData transactions

[evan-forbes]

Currently, when paying for a message to be included on chain, we add padding to the message before sending it by default. We should remove this padding from the messages, as block producers can simply add it when they are calculating the data hash. As discussed in #431, we can still pay for padding even after we remove it from the message.

This will reduce the size of the message when passed over a p2p network, cached in a mempool, or stored in the block data base/transaction index.

When we begin actually enforcing that the message share commitments are a subtree root of the data hash, and using the non-interactive defaults #382, this will cause even more padding to be necessary, which amplifies the issues of including padding in the actual message.