Remove the RemoveParticipant protocol for having new DKGs specify the participants which were removed

Obvious code cleanup is obvious.

substrate/abi/src/validator_sets.rs

@@ -8,15 +8,10 @@ use serai_validator_sets_primitives::*;
 pub enum Call {
   set_keys {
     network: NetworkId,
+    removed_participants: Vec<SeraiAddress>,
     key_pair: KeyPair,
     signature: Signature,
   },
-  remove_participant {
-    network: NetworkId,
-    to_remove: SeraiAddress,
-    signers: Vec<SeraiAddress>,
-    signature: Signature,
-  },
   allocate {
     network: NetworkId,
     amount: Amount,