Remove unused delay-shutdown REST endpoint

[azmkercso]

Issue #, if available:

• P398067552

Description of changes:

• Remove the /delay-shutdown HTTP endpoint and its backing _delayShutdown() method from remoteExtensionHostAgentServer.ts
• This feature is not used by the service — the shutdown lifecycle is fully managed by extension host connection events (_onDidCloseExtHostConnection)
• The endpoint was positioned before the connection token validation, meaning it required no authentication
• After this change, GET /delay-shutdown returns 403 (without token) or 404 (with token) instead of 200

Testing:

• Verified all existing patches apply cleanly with the new patch appended (quilt push -a succeeds)
• Confirmed no references to delay-shutdown or _delayShutdown remain in the patched source
• Confirmed _cancelShutdown() and _waitThenShutdown() are preserved (still used by _onDidCloseExtHostConnection)

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