Remove deprecated legacy gossip handler

[ceyonur]

We have deprecated legacy gossip handler in favor of p2p sdk gossip handler (push/pull gossips). We have removed it from coreth with the PR ava-labs/coreth#706

Coreth PR is a big one but these changes should be same to remove the deprecated handler from subnet-evm as well:

• Network file: https://github.com/ava-labs/coreth/pull/706/files#diff-9cd76a0999351a7520dce33ca95e771d39a4560eb9f589abd38d392506fdfc39

• Network test file: https://github.com/ava-labs/coreth/pull/706/files#diff-305c10713f73ecc40109c234bebfcdcac4bfc4a32a0e4a894e04bc8a0d3b1635

• Handler file removal: https://github.com/ava-labs/coreth/pull/706/files#diff-99c1d8c878c89edcd40d329c34104fd12df71d1bdeb3dfae51ad7f5758d42424

• Gossip stats removal: https://github.com/ava-labs/coreth/pull/706/files#diff-956126bb535d3b8f9fe540954850254433316fabe9e908b55a48de6a1cb153c1

• Skip registration for EthTxsGossip type in codec: https://github.com/ava-labs/coreth/pull/706/files#diff-295ec2bc5b901db96bc911c27f3ef2df3eaf51e592081cab92d1dd51cbe372bc

• no-op handler changes: https://github.com/ava-labs/coreth/pull/706/files#diff-f89e2ab248338043520f9f69119ec289998cdca066fa80e0d86160cfe00717dd

• message, message_test, handler_test removal: https://github.com/ava-labs/coreth/pull/706/files#diff-a447f410a435eacd25a234db3635e85c4664c11c7feb7326556b62662fc5405e