crypto/tls: remove flaky cancellation test

crypto/tls: remove flaky cancellation test

This will be reintroduced again once the source of the
flakiness has been determined and fixed.

Fixes #45084

Change-Id: I6677b27fcd71e8c9bb8edbe8e3be70e5a271ebd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/302569
Trust: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>