Revert "UrfDeviceOfono: explicitly set DBus method call timeout to 30 seconds (30000 ms)"

This reverts commit ff316570e694c0ed7444e1030d49008aa0d4c5ce.
UrfConfig: always write persistence file on change

Always write the persistence file out to disk when the persist state
is changed for a device type. This avoids issues with inconsistency if
writing the file fails on shutdown.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>