net/http/cgi: Remove hard-coded ServeHTTP timeout

Close #43624

Change-Id: Ifaea3d8ec2aeabbd923abf5edd7497172dbf855a
GitHub-Last-Rev: ea3ef953a1dbbb07c4554e4062571af716e9505b
GitHub-Pull-Request: golang/go#43803
Reviewed-on: https://go-review.googlesource.com/c/go/+/284778


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Trust: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>