packagekit: Remove remaining mentions of yum
packagekit: Remove remaining mentions of yum

CentOS/RHEL 8 and Fedora use dnf, and we don't support CentOS/RHEL 7
from master any more, so these can go. There is no functional change.

Closes #13832
cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit