Drop dependency in pkexec
For the reasons described in gnome-control-center#1604 (closed)
We are spawing pkexec here https://gitlab.gnome.org/GNOME/gnome-system-monitor/-/blob/d80dceedd106ca2415aeb0cb71b54ae4bd93bf75/src/gsm_pkexec.cpp#L14
For gnome-control-center there's a merge-request where an alternative using polkit is used. See https://gitlab.gnome.org/GNOME/gnome-control-center/-/blob/3b2910b821382a69dc92217fe69228a511539a30/panels/sharing/cc-remote-login.c#L135