x/pkgsite: don't tee /_ah/* requests from godoc.org

[julieqiu]

As described in #39702, the teeproxy was implemented as a way to forward requests from godoc.org to pkg.go.dev and ensure that pkg.go.dev can handle the current workload of godoc.org.

We shouldn't tee any requests from AppEngine, such as /_ah/ready.

A filter was previously implemented here:
https://github.com/golang/gddo/blob/051695c33a3fcf220bffc5815eebc4b0a346c983/gddo-server/main.go#L1042

However, we are still seeing these requests via the teeproxy.

/cc @shaqque

[gopherbot]

Change https://golang.org/cl/249438 mentions this issue: internal/teeproxy: don't tee App Engine requests