gdm: remove /var/run from package

The /var/run/gdm and /var/run/gdm/greeter directories are created by
the daemon if they don't exist. The /var/run directory is already
created by base-files.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>