chore: remove merge transition code (#8680)

**Motiviation**

All networks have completed the merge transition and most execution
clients no longer support pre-merge so it's not even possible anymore to
run a network from a genesis before bellatrix, unless you keep it to
phase0/altair only, which still works after this PR is merged.

This code is effectively tech debt, no longer exercised and just gets in
the way when doing refactors.

**Description**

Removes all code related to performing the merge transition. Running the
node pre-merge (CL only mode) is still possible and syncing still works.
Also removed a few CLI flags we added for the merge specifically, those
shouldn't be used anymore. Spec constants like
`TERMINAL_TOTAL_DIFFICULTY` are kept for spec compliance and ssz types
(like `PowBlock`) as well. I had to disable a few spec tests related to
handling the merge block since those code paths are removed.

Closes https://github.com/ChainSafe/lodestar/issues/8661

packages/params/src/index.ts

@@ -271,6 +271,7 @@ export const MAX_REQUEST_LIGHT_CLIENT_COMMITTEE_HASHES = 128;
 
 /**
  * Optimistic sync
+ * @deprecated All networks have completed the merge transition, blocks are always safe to import optimistically.
  */
 export const SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY = 128;
 /** @deprecated */