Overview

As discussed recently, we're not going with the type descriptor approach from #622. Instead, splitting the improvements into targeted PRs, as #622 even after the cleanup is a bit too much, as most likely we don't want to include all changes (even after the cleanup), so this approach should work better.
This is the first one - removing cleanup infrastructure that became dead code after #635 and #636.

What changed

After #635 (string lower no longer needs cleanup) and #636 (fixed JSObject? double-release in struct lowering), no type produces cleanup code at runtime. This PR removes all of it:

• cleanupCode printer field from IntrinsicJSFragment.PrintCodeContext
• tmpStructCleanups array and swift_js_struct_cleanup BJS handler from generated JS
• _swift_js_struct_cleanup extern and wrapper from BridgeJSIntrinsics.swift
• swift_js_struct_cleanup stub from instantiate.js
• Struct helper lower() no longer returns { cleanup } - just pushes fields to stacks
• Enum helper lower() returns caseId directly instead of { caseId, cleanup }
• Struct/enum helper factory collapse: () => { return () => ({...}); } simplified to () => ({...}); call-site ()() becomes ()
• init(unsafelyCopying:) no longer calls struct cleanup; lower extern returns Void instead of Int32
• strStack.push(textDecoder.decode(bytes)) inlined (removed intermediate variable)