Remove unused functions from src/udev/{label,mkdir}

We remove the following unused functions:

  label_socket_set
  label_socket_clear
  label_free
  label_get_create_label_from_exe
  label_retest_selinux
  label_bind
  mkdir_label
  mkdir_safe
  mkdir_safe_label
  mkdir_p_label

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>