docs: hw-vuln: tsx_async_abort.rst: get rid of an unused ref

docs: hw-vuln: tsx_async_abort.rst: get rid of an unused ref

The virt_mechanism reference there points to a section
called elsewhere (Virtualization mitigation). Also, it is
not used anywere.

Besides that, it conflicts with a label with the same name
inside:

	Documentation/admin-guide/hw-vuln/mds.rst

Perhaps added due to some cut-and-paste?

Anyway, as this is not used, let's just get rid of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/681c8e2916bf4943ac2277f181668bfbc5fdbc01.1583250595.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>