http2: delete README

Out of date, redundant with package doc, just drop it.

Fixes golang/go#47624.

Change-Id: Ic4a686330d69744e57bf01785c53707ba29944b9
Reviewed-on: https://go-review.googlesource.com/c/net/+/341369
Trust: Damien Neil <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: neild