GovPreventSpamDecorator is no longer needed

[mpoke]

Problem

SDK 0.47 introduced SDK 0.47 introduced the MinInitialDepositRatio param to the gov module (see cosmos/cosmos-sdk#12771). This means we no longer need the GovPreventSpamDecorator in Gaia. The same check is done when handling a SubmitProposal message, see here.

Closing criteria

• Remove GovPreventSpamDecorator
• Initialize the MinInitialDepositRatio param of the gov module to sdk.NewDecWithPrec(10, 2), see https://github.com/cosmos/cosmos-sdk/blob/v0.47.8/proto/cosmos/gov/v1/gov.proto#L210