mqtt: Remove unused mqtt_header_state_t
mqtt: Remove unused mqtt_header_state_t

mqtt_header_state_t was added by commit 0450bd009397 ("MQTT Client:  Added support for receiving empty payload"),
but it's no longer used since commit fd564b1f1733 ("client receive: refactor receive to read only one message to fragment only payload for longer messages").
Thus remove the unused mqtt_header_state_t.

Signed-off-by: Axel Lin <b6ffd6973e972cb999e8e535ab74da7fee0c035f@ingics.com>

tuanpmt/espmqtt