Unnecessary field option in staking/v1beta1/authz.proto

[hallazzang]

Since max_tokens is not a repeated field, [(gogoproto.castrepeated) = "github.com/cosmos/cosmos-sdk/types.Coin"] doesn't have any effect:

cosmos-sdk/x/staking/proto/cosmos/staking/v1beta1/authz.proto

Line 19 in da5a74d

cosmos.base.v1beta1.Coin max_tokens = 1 [(gogoproto.castrepeated) = "github.com/cosmos/cosmos-sdk/types.Coin"];

Also the option value seems incorrect, too. It supposed to be github.com/cosmos/cosmos-sdk/types.Coins if max_tokens were a repeated field. We should remove this unnecessary gogoproto field option.