Fix markdown usage on rhel6.
Fix markdown usage on rhel6.

The version of python-markdown on rhel6 is too old and doesn't know
about the ``html_replacement_text`` argument.

By removing it like we're doing here, do we open ourselves up to attack?
No.  If we were embedding comments from *any user* from bodhi, yes.
They could inject JS-bearing HTML in a comment, that could do malicious
things to our users over here.  However, we're not embedding comments
from just *any* user -- only from the *bodhi* system user, which we can
trust.

fedora-infra/fedora-packages,fedora-infra/fedora-packages,fedora-infra/fedora-packages,fedora-infra/fedora-packages