Remove /api proxy

[mattvb91]

Currently every request that goes to /api triggers the nextjs lambda which then in turn proxies a request out to the "real" PHP lambda.
So for every request coming in we are actually using 2 lambdas

Once we can override custom paths to a custom origin this issue should be fixed and all /api requests land directly on the php lambda

serverless-nextjs/serverless-next.js#892

• remove http-proxy-middleware & @types/http-proxy-middleware packages
• Remove /api page
• Add route into frontend/serveless.yml config to route to php lambda