perf: state mutability and unused var

[gosuto-inzasheru]

Changes in PR:

• perf: restrict state mutability to pure
• perf: remove unused variable

implement performance recommendations from solidity compiler:

Warning: Unused local variable.
  --> contracts/test/TestFallbackReceiver.sol:13:24:
   |
13 |         (bool success, bytes memory data) = ethReceiver.call{value: address(this).balance}("");
   |                        ^^^^^^^^^^^^^^^^^


Warning: Function state mutability can be restricted to pure
   --> contracts/SafeProtocolRegistry.sol:122:5:
    |
122 |     function supportsInterface(bytes4 interfaceId) external view override returns (bool) {
    |     ^ (Relevant source part starts here and spans across multiple lines).


Warning: Function state mutability can be restricted to pure
   --> contracts/SignatureValidatorManager.sol:226:5:
    |
226 |     function supportsInterface(bytes4 interfaceId) external view override returns (bool) {
    |     ^ (Relevant source part starts here and spans across multiple lines).


Warning: Function state mutability can be restricted to pure
  --> contracts/test/TestPlugin.sol:21:5:
   |
21 |     function supportsInterface(bytes4 interfaceId) external view override returns (bool) {
   |     ^ (Relevant source part starts here and spans across multiple lines).