net/http: simplify validPort after CL 189258

[bradfitz]

https://go-review.googlesource.com/c/go/+/189258 pushes some URL validation down into net/url that was previously only done in net/http.

We can remove net/http's at this point.

/cc @FiloSottile

[gopherbot]

Change https://golang.org/cl/190057 mentions this issue: net/http: remove redundant port validation