winfsnotify: delete package

Users who still need to resolve imports of this package can require
the last extant version of the module carved out in the prior CL.

Also delete fsnotify/README.txt. The substitute import path it
suggests is no longer accurate, and users have had seven years(!) to
migrate.

Fixes golang/go#51447.
Fixes golang/go#49734.

Change-Id: Ibf7e4566b65dd3927083d1516b0a53324ce1c4ad
Reviewed-on: https://go-review.googlesource.com/c/exp/+/389615
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: Bryan C. Mills