context: delete lone example

This is motivated by a few reasons. One, the upstream package has more
examples, and no one should be looking at this old package to learn how
to use it. Seeing an example might make it seem like the scope of the
documentation here is to provide examples, and that there aren't many
of them. Instead of trying to add more examples or maintain the current
one by porting the de-flake enhancement from CL 460999, delete the only
example here.

Second, running 'go fix ./...' causes the 'context' fix to rewrite the
import path of the example from "golang.org/x/net/context" to "context".
That is a false positive in the fix, and I would've liked it fix the
fix, but it only has the AST information at this time, not type info,
so the import path isn't currently available to the check. That means
it can't know when it's running on the golang.org/x/net/context package,
which is the one place it should skip the rewrite.

It seems simpler to just delete the example, and then it becomes
possible to use 'go fix ./...' safely on the entire x/net module.

For golang/go#49506.

Change-Id: I97eba33ca2e1f2960aef8340d8b561639a26ee48
Reviewed-on: https://go-review.googlesource.com/c/net/+/650156
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur