Python provision: remove "ExistingBackend"

Python provision: remove "ExistingBackend"

This was unused and broken. e.g. here:

-    def init(self):
-        # Check to see that this 'existing' LDAP backend in fact exists
-        ldapi_db = Ldb(self.ldapi_uri)

there is no attribute self.ldapi_uri, so this would always raise an
exception.

It was being left around in case it became useful, but that doesn't
seem to be happening.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Noel Power <noel.power@suse.com>