Deprecate and phase out broadcast mode block.

[greg-szabo]

As mentioned in the below issue, broadcast mode block is deprecated. It should be removed, possibly in a granular manner. I'd start with removing it from the help section so people don't start using it.
Maybe a warning should be emitted in an newer release when a user explicitly sets it.
Eventually, it should be completely removed.

This is fair, but note, broadcast mode block is deprecated and should not really be used at all.

Originally posted by @alexanderbez in #12144 (comment)

[alexanderbez]

I would just completely remove it outright and not backport it, such that the next release only has sync and async modes.

[alexanderbez]

@julienrbrt or @facundomedica, might be a fun task to tackle 🙌

[tac0turtle]

does it matter that sync and async are the same in the code of tendermint. @cmwaters mentioned it to me

[alexanderbez]

What do you mean the same? async just sends the transaction w/o waiting for CheckTx

[cmwaters]

What do you mean the same? async just sends the transaction w/o waiting for CheckTx

Yeah that's what I thought until I was looking at the code recently. Seems in v0.36 async and sync were going to be the same but we've filed an issue and will hopefully fix it soon.

[cmwaters]

I think in v0.37 we will look to simplify broadcasting txs to a single method. It's redundant to add the asynchronous method when txs can already be submitted in parallel.

[alexanderbez]

Agreed, It should just be sync, i..e CheckTx is executed only. This covers 99% of use-cases.