perf: refactor apk modifiers to use internal functions (#268)

* fix: deprecated struct field for earning receiver

* perf: move modifier logic to internal function

src/BLSApkRegistry.sol

@@ -12,10 +12,7 @@ contract BLSApkRegistry is BLSApkRegistryStorage {
 
     /// @notice when applied to a function, only allows the RegistryCoordinator to call it
     modifier onlyRegistryCoordinator() {
-        require(
-            msg.sender == address(registryCoordinator),
-            "BLSApkRegistry.onlyRegistryCoordinator: caller is not the registry coordinator"
-        );
+        _checkRegistryCoordinator();
         _;
     }
 
@@ -281,4 +278,11 @@ contract BLSApkRegistry is BLSApkRegistryStorage {
     function getOperatorId(address operator) public view returns (bytes32) {
         return operatorToPubkeyHash[operator];
     }
+
+    function _checkRegistryCoordinator() internal view {
+        require(
+            msg.sender == address(registryCoordinator),
+            "BLSApkRegistry.onlyRegistryCoordinator: caller is not the registry coordinator"
+        );
+    }
 }