Put back setGroupMembers() to internal

Setting it public was done previously to reduce the contract size which is
not relevant anymore.

Author: ngrinkevich

solidity/contracts/libraries/operator/Groups.sol

@@ -76,7 +76,7 @@ library Groups {
         bytes memory groupPubKey,
         address[] memory members,
         bytes memory misbehaved
-    ) public {
+    ) internal {
         self.groupMembers[groupPubKey] = members;
 
         // Iterate misbehaved array backwards, replace misbehaved