shell: Clean up PatternFly 3 leftover
shell: Clean up PatternFly 3 leftover

blank-slate-pf and the FontAwesome icon both don't work any more in the
PF4-only shell. No real-life user should ever see this fallback any more
these days, having an icon there is really not important. So just drop
the icon and blank-slate style.

cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit