chore: Delete dead lockdown code (#3841)

Remove dead lockdown code from when we were manually running lockdown.
This is no longer the case as the execution environments use LavaMoat.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: deletes a dead, unused helper and only affects code hygiene;
remaining lockdown hardening continues to be handled via LavaMoat plus
existing `executeLockdownMore`/`executeLockdownEvents` calls.
> 
> **Overview**
> Removes the unused SES `executeLockdown` helper
(`src/common/lockdown/lockdown.ts`), leaving lockdown initiation to
LavaMoat and retaining only the extra hardening steps
(`executeLockdownMore` and `executeLockdownEvents`) invoked by the
execution environment entrypoints.
> 
> Updates `coverage.json` thresholds to reflect the small change in
measured line/statement coverage.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
31950ca. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: FrederikBolding

packages/snaps-execution-environments/coverage.json

@@ -1,6 +1,6 @@
 {
   "branches": 90.09,
   "functions": 95.34,
-  "lines": 91.7,
-  "statements": 90.68
+  "lines": 92.74,
+  "statements": 91.69
 }