Make city field of board non-required

Make city field of board non-required so users can ignore it and
it will be automatically set based on their group membership
by ``onboard_types_node_presave()``.

Addresses #3484
onboard_types_node_presave gets node argument by reference

hook_node_presave() receives the node by reference.  So, just
alter the node in the type-specific calls rather than returning
the node and assigning the return value.

See
https://api.drupal.org/api/drupal/modules!node!node.api.php/function/hook_node_presave/7

Addresses #3484