udevadm: drop pointless must_be_root() checks

Checking if we are root on the client side is generally pointless, since the
privileged operation will fail anyway and we can than log what precisly went
wrong.

A check like this makes sense only if:
- we need to do some expensive unprivileged operation before attempting the
  privileged operation, and the check allows us avoid wasting resources.
- the privileged operation would fail but in an unclear way.

Neither of those cases applies here.

This fixes calls like 'udevadm control -h' as unprivileged user.

src/udev/udevadm-control.c

@@ -73,10 +73,6 @@ int control_main(int argc, char *argv[], void *userdata) {
                 {}
         };
 
-        r = must_be_root();
-        if (r < 0)
-                return r;
-
         if (running_in_chroot() > 0) {
                 log_info("Running in chroot, ignoring request.");
                 return 0;

src/udev/udevadm-trigger.c

@@ -301,12 +301,6 @@ int trigger_main(int argc, char *argv[], void *userdata) {
                 }
         }
 
-        if (!arg_dry_run || ping) {
-                r = must_be_root();
-                if (r < 0)
-                        return r;
-        }
-
         if (ping) {
                 _cleanup_(udev_ctrl_unrefp) struct udev_ctrl *uctrl = NULL;