[Feature]: Remove DecProto

[robert-zaremba]

Problem Definition

DecProto is not used anywhere, and I've seen some project using it instead of Dec type (which correctly implements Protobuf).

Proposal

To avoid confusions, we can simply remove DecProto.

Considerations

Advantage of having DecProto is the ability to write in the proto files:

cosmos.base.v1beta1.DecProto dec_amount = 1;

Disadvantages: more code and confusions in Go side of having Dec and DecProto.

[robert-zaremba]

Happy to handle it if others agree with the proposal.

[julienrbrt]

As it is not used, and it allows us to remove more things in types/math.go (#14405 (comment)), personally I do not see why not.
cc @aaronc.

[julienrbrt]

Happy to handle it if others agree with the proposal.

Do you still want to submit a PR? (ref #16798 (comment))

[robert-zaremba]

Sure, I can remove IntProto and DecProto. So it's confirmed that we are good to do it?