Trust get_preferred_snmp_management_profile

Netbox.get_preferred_snmp_manage_profile() should be trusted to return
a proper SNMP profile.  I see no reason to guard against faulty
return values like this.

python/nav/portadmin/snmp/base.py

@@ -21,6 +21,7 @@
 from typing import Dict, Sequence, List, Any
 
 from nav import Snmp
+from nav.Snmp.profile import get_snmp_session_for_profile
 from nav.Snmp import safestring, OID
 from nav.Snmp.errors import (
     UnsupportedSnmpVersionError,
@@ -174,10 +175,6 @@ def _get_read_only_handle(self):
 
             if not profile:
                 raise NoReadOnlyManagementProfileError
-            if not hasattr(profile, "snmp_community") or not hasattr(
-                profile, "snmp_version"
-            ):
-                raise InvalidManagementProfileError
 
             self.read_only_handle = Snmp.Snmp(
                 host=self.netbox.ip,