take-over-existing-cluster: do not call var_files

take-over-existing-cluster: do not call var_files

We were using var_files long ago when default variables were not in
ceph-defaults, now the role exists this is not need. Moreover having
these two var files added:

- roles/ceph-defaults/defaults/main.yml
- group_vars/all.yml

Will create collision and override necessary variables.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1555305
Signed-off-by: Sébastien Han <seb@redhat.com>
(cherry picked from commit b7387068109a521796f8e423a61449541043f4e6)