nat: correct pool ops when disable nat44 plugin

Type: fix

Replace vec ops with pool ops when iterating over pool of
interfaces in plugin_disable() functions.
With this change plugin disabling function and interface role
disabling functions iterate over identical sets of registered
interfaces, preventing 'interface couldn't be found' errors.

Signed-off-by: Alexander Skorichenko <askorichenko@netgate.com>
Change-Id: I4e191bcc5322ff1f9f7730c6fda5e6af86a8eec9