Simplify staking reward query

[ethanfrey]

See https://github.com/CosmWasm/wasmd/blob/v0.14.0/x/wasm/internal/keeper/query_plugins.go#L269-L292

cosmos/cosmos-sdk#7466 was never closed but the relevant code was made public in stargate, so we can now use these lower level calls, rather than going through the public query interface.

[ethanfrey]

Trying to resolve this, I found that it is much harder to use the lower-level primitive, and the higher-level one has been cleaned up in the meantime. We'd have to more or less copy all the logic anyway, load validators and delegators, etc.

https://github.com/cosmos/cosmos-sdk/blob/v0.42.2/x/distribution/keeper/grpc_query.go#L119-L157

I will just stick with the current approach.