Remove unused flag in gov proposals

[jhernandezb]

flagProposalType is being set in all gov proposals but only used in the sdk for the main submit-proposal command https://github.com/cosmos/cosmos-sdk/blob/v0.45.4/x/gov/client/cli/parse.go#L13

We should consider removing it if it's not actually used.

Context: #796 (comment)

[alpe]

I remember that this was used when reading a json file for the gov proposal, once. But this may not be the case anymore 🤷

[ethanfrey]

Adding it to v0.28.0.

If this is possible without breaking the json import, let's do it. If not, let's just leave it as is (and close as "wont fix")