community/xdm: Remove /usr/ucb from system/user path

Since f440e2db, /etc/profile appends to $PATH instead of overwriting it. The default PATH used by xdm includes /usr/ucb which we don't use on Alpine. Since /etc/profile no longer overwrites $PATH, /usr/ucb will be included in the users $PATH variable unless it's explicitly overwritten by the users configuration, even though Alpine doesn't have that directory.