Handle removal of dnf.repo._md_expire_cache() in DNF 3.4

DNF 3.4 removed _md_expire_cache(), which we were still using.
Let's follow how upstream replaced it, by using the expire()
method of the underlying librepo repo object...unfortunately
this is still using private interfaces, but I don't see a way
to do it using public ones.

Signed-off-by: Adam Williamson <awilliam@redhat.com>