[eth-json-rpc-middleware] fix: remove condition to allow 'cosmos' as valid value for verifyingContract field in typed signatures

We have added code to allow `cosmos` as valid `verifyingContract` value for typed signatures. The reason for this is to keep support signature signing on cosmos network.

A lot of cosmos dapps use this deprecated sdk with hard coded value of `verifyingContract`: https://github.com/evmos/ethermint/blob/main/ethereum/eip712/domain.go#L29

Once users stop using this SDK we can remove this hard coded condition.

Ref: https://github.com/MetaMask/metamask-extension/issues/26980

Start in the eth-json-rpc-middleware package by locating validation for the verifyingContract field and read the linked Ethermint SDK reference. Confirm the intended handling of the deprecated hard-coded cosmos value, then run the package's existing checks; done means Cosmos signature signing remains supported with the requested condition change.

Written by the indexing model from the issue text.