net/mqtt: combine mqtt_parser for PUBLISHER and SUBSCRIBER

Currently, app type MQTT_APP_PUBLISHER_SUBSCRIBER is not supported.
mqtt_init() will return EINVAL because the context rcv function is
either mqtt_publisher_parser or mqtt_subscriber_parser (not both).

Let's combine these functions into mqtt_parser and remove the app
type check from mqtt_init().

Change-Id: I60460e011395864706e293c997e9f8a65681b368
Signed-off-by: Michael Scott <michael.scott@linaro.org>