net/http: do not cancel request context on response body read

When sending a Request with a non-context deadline, we create a
context with a timeout. This context is canceled when closing the
response body, and also if a read from the response body returns
an error (including io.EOF).

Cancelling the context in Response.Body.Read interferes with the
HTTP/2 client cleaning up after a request is completed, and is
unnecessary: The user should always close the body, the impact
from not canceling the context is minor (the context timer leaks
until it fires).

Fixes #49366.

Change-Id: Ieaed866116916261d9079f71d8fea7a7b303b8fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/361919


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>