mgr/dashboard: fix nfs exports form issues with squash field

mgr/dashboard: fix nfs exports form issues with squash field

1. Squash is not a mandatory field.
2. If the export is created from backend, then squash can have many
   different names for each types of squash. for eg.
```
['root', 'root_squash', 'rootsquash', 'rootid', 'root_id_squash', 'rootidsquash', 'all', 'all_squash', 'allsquash', 'all_anomnymous', 'allanonymous', 'no_root_squash', 'none', 'noidsquash']
```
so we need a proper matching in the ui too otherwise the edit field will
not have any value for the squash field.

3. Removed unncessary dropdown helper if there are values for the squash
   and access types.

Fixes: https://tracker.ceph.com/issues/57114
Signed-off-by: Nizamudeen A <nia@redhat.com>