refactor : remove unused params in emssions module

[kingpinXD]

Describe the Issue Current emissions params

message Params {
  option (gogoproto.goproto_stringer) = false;
  string max_bond_factor = 1;
  string min_bond_factor = 2;
  string avg_block_time = 3;
  string target_bond_ratio = 4;
  string validator_emission_percentage = 5;
  string observer_emission_percentage = 6;
  string tss_signer_emission_percentage = 7;
  string duration_factor_constant = 8;
  string observer_slash_amount = 9 [
    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Int",
    (gogoproto.nullable) = false
  ];
}

A lot of these params are not used and the struct can be condensed to

message Params {
  option (gogoproto.goproto_stringer) = false;
  string validator_emission_percentage = 1;
  string observer_emission_percentage = 2;
  string tss_signer_emission_percentage = 3;
  string observer_slash_amount = 4 [
    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Int",
    (gogoproto.nullable) = false
  ];
}

Expected Outcome Emissions params is refactored into the the suggested structure

[lumtis]

I think this can be addressed in the same PR as https://github.com/zeta-chain/node/issues/1813

Also we have a hardcoded value for the rewards per block. I think we should change it into a param?

[kingpinXD]

Related : https://github.com/zeta-chain/protocol-private/issues/96