`@remotion/lambda`: Remove merge handler

[JonnyBurger]

• Will not be supported when we move to Lambda streaming
• Merging is now a lot faster so it's way more unlikely this code path is hit

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Name	Status	Preview	Comments	Updated (UTC)
bugs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 23, 2024 8:28pm
remotion	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 23, 2024 8:28pm