Remove references to vote extensions from Tendermint spec before releasing v0.36

[sergio-mena]

Summary

Tendermint v0.36 is likely to not implement vote extensions, as it implies modifying the block format, in particular, CommitSig (see #7664)

For the time being, we leave the ABCI++ specification as it is: complete with vote extensions, so that everyone can we what we are planning to implement.

Whenever we "pull the throttle" for v0.36, we need to remove the references to vote extensions from that version of the spec, so as to avoid confusion.

Dependencies

• v0.36 of the spec branch frozen (throttle branch). Alternatively, throttle branch for v0.36 in the Go repo if the spec is finally merged with the Go implementation.

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[sergio-mena]

Closing this because of the change of plans (vote extensions to be released with v0.36)