service_spec: force ceph-exporter sock_dir to be unest or "/var/run/ceph/"

service_spec: force ceph-exporter sock_dir to be unest or "/var/run/ceph/"

As discussed in https://tracker.ceph.com/issues/69475 this
setting is effectively useless as it only controls the directory
inside the container where the ceph-exporter will write out
its asok file, and has no influence over where it is on the
host where the ceph-exporter daemon is deployed. Given any
custom values for the sock_dir setting would have always
been broken, we decided to skip out on writing a proper migration
step in cephadm to deal with this, and instead just force this
field to be unset

Signed-off-by: Adam King <adking@redhat.com>