Ensure deposit signatures do not use aggregate functions (#1935)

## Issue Addressed

Resolves #1333 

## Proposed Changes

- Remove `deposit_signature_set()` function
- Prevent deposits from being in `SignatureSets`
- User `Signature.verify()` to verify deposit signatures rather than a signature set which uses `fast_aggregate_verify()`

## Additional Info

n/a

consensus/state_processing/src/per_block_processing/verify_deposit.rs

@@ -1,7 +1,5 @@
 use super::errors::{BlockOperationError, DepositInvalid};
-use crate::per_block_processing::signature_sets::{
-    deposit_pubkey_signature_message, deposit_signature_set,
-};
+use crate::per_block_processing::signature_sets::deposit_pubkey_signature_message;
 use merkle_proof::verify_merkle_proof;
 use safe_arith::SafeArith;
 use tree_hash::TreeHash;
@@ -17,11 +15,11 @@ fn error(reason: DepositInvalid) -> BlockOperationError<DepositInvalid> {
 ///
 /// Spec v0.12.1
 pub fn verify_deposit_signature(deposit_data: &DepositData, spec: &ChainSpec) -> Result<()> {
-    let deposit_signature_message = deposit_pubkey_signature_message(&deposit_data, spec)
+    let (public_key, signature, msg) = deposit_pubkey_signature_message(&deposit_data, spec)
         .ok_or_else(|| error(DepositInvalid::BadBlsBytes))?;
 
     verify!(
-        deposit_signature_set(&deposit_signature_message).verify(),
+        signature.verify(&public_key, msg),
         DepositInvalid::BadSignature
     );