bug: delete unused submodule folder vercel-app

[noy4]

delete folder gateway/cookbook/integrations/vercel/vercel-app to enable to use this repo as submodule

Description:
We tried to use this gateway as submodule in our project and deployed it to Cloudflare Pages, but it failed. It seems it's because of the unused vercel-app folder. Error log:

...

2024-10-28T06:32:59.719117Z	Submodule 'gateway' (https://github.com/Portkey-AI/gateway.git) registered for path 'gateway'
2024-10-28T06:32:59.719667Z	Cloning into '/opt/buildhome/clone/gateway'...
2024-10-28T06:32:59.71987Z	Submodule path 'gateway': checked out 'fa2536760103b2707c5cf80546f6cc57a9051ada'
2024-10-28T06:32:59.719959Z	fatal: No url found for submodule path 'gateway/cookbook/integrations/vercel/vercel-app' in .gitmodules
2024-10-28T06:32:59.720019Z	fatal: Failed to recurse into submodule path 'gateway'
2024-10-28T06:32:59.720076Z	
2024-10-28T06:33:00.490148Z	Failed: error occurred while updating repository submodules