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/signature_sets.rs

@@ -288,8 +288,6 @@ where
 }
 
 /// Returns the BLS values in a `Deposit`, if they're all valid. Otherwise, returns `None`.
-///
-/// This method is separate to `deposit_signature_set` to satisfy lifetime requirements.
 pub fn deposit_pubkey_signature_message(
     deposit_data: &DepositData,
     spec: &ChainSpec,
@@ -301,18 +299,6 @@ pub fn deposit_pubkey_signature_message(
     Some((pubkey, signature, message))
 }
 
-/// Returns the signature set for some set of deposit signatures, made with
-/// `deposit_pubkey_signature_message`.
-pub fn deposit_signature_set(
-    pubkey_signature_message: &(PublicKey, Signature, Hash256),
-) -> SignatureSet {
-    let (pubkey, signature, message) = pubkey_signature_message;
-
-    // Note: Deposits are valid across forks, thus the deposit domain is computed
-    // with the fok zeroed.
-    SignatureSet::single_pubkey(signature, Cow::Borrowed(pubkey), *message)
-}
-
 /// Returns a signature set that is valid if the `SignedVoluntaryExit` was signed by the indicated
 /// validator.
 pub fn exit_signature_set<'a, T, F>(