Remove custom protobuf Timestamp from tendermint-proto

[greg-szabo]

https://github.com/informalsystems/tendermint-rs/blob/7eaa81bddd764ab76e68fce0bb2bcd094168bedd/proto/src/protobuf.rs#L1-L4

Prost now has a way to not include comments of the built-in types in the generated code (which caused doctest issues).

Remove the custom Timestamp and Duration implementation.

Benefit: the built-in prost types have conversion functions for DateTime and other types.