Fix duplicate registration of feemarket GenesisState and Params

[nddeluca]

Description

v0.12.2 introduced legacy types which attempt to register on init, conflicting with the existing types.

 > ethermintd version
2022/03/30 23:10:02 proto: duplicate proto type registered: ethermint.feemarket.v1.Params
2022/03/30 23:10:02 proto: duplicate proto type registered: ethermint.feemarket.v1.GenesisState
0.12.2

This also results in the grpc server showing an incorrect description of the Params type.

 > grpcurl -plaintext localhost:9090 describe .ethermint.feemarket.v1.Params
ethermint.feemarket.v1.Params is a message:
message Params {
  bool no_base_fee = 1;
  uint32 base_fee_change_denominator = 2;
  uint32 elasticity_multiplier = 3;
  int64 initial_base_fee = 4;
  int64 enable_height = 5;
}

The proto registration isn't needed for migrations, so we should be fine in just removing the init calls in the legacy types.

This PR fixes the warnings on init, and results in the correct registration for the feemarket Params.

> grpcurl -plaintext localhost:9090 describe .ethermint.feemarket.v1.Params
ethermint.feemarket.v1.Params is a message:
message Params {
  bool no_base_fee = 1;
  uint32 base_fee_change_denominator = 2;
  uint32 elasticity_multiplier = 3;
  int64 enable_height = 5;
  string base_fee = 6 [(.gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Int", (.gogoproto.nullable) = false];
  reserved 4;
  reserved "initial_base_fee";
}

---

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• Added relevant godoc comments.
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---

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[codecov]

Codecov Report

Merging #1023 (a01ac42) into main (7d11c93) will increase coverage by 0.16%.
The diff coverage is n/a.

❗ Current head a01ac42 differs from pull request most recent head ebd2a23. Consider uploading reports for the commit ebd2a23 to get more accurate results

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
+ Coverage   59.16%   59.32%   +0.16%     
==========================================
  Files          80       80              
  Lines        6577     6577              
==========================================
+ Hits         3891     3902      +11     
+ Misses       2467     2456      -11     
  Partials      219      219              

Impacted Files	Coverage Δ
x/feemarket/migrations/v010/migrate.go	85.41% <0.00%> (+22.91%)	⬆️