Remove DB migrations for legacy database schemas (#3181)

## Proposed Changes

Remove support for DB migrations that support upgrading from schema's below version 5. This is mostly for cosmetic/code quality reasons as in most circumstances upgrading from versions of Lighthouse this old will almost always require a re-sync.

## Additional Info

The minimum supported database schema is now version 5.

beacon_node/beacon_chain/src/errors.rs

@@ -91,7 +91,7 @@ pub enum BeaconChainError {
     BlockSignatureVerifierError(state_processing::block_signature_verifier::Error),
     BlockReplayError(BlockReplayError),
     DuplicateValidatorPublicKey,
-    ValidatorPubkeyCacheFileError(String),
+    ValidatorPubkeyCacheError(String),
     ValidatorIndexUnknown(usize),
     ValidatorPubkeyUnknown(PublicKeyBytes),
     OpPoolError(OpPoolError),