MMS: Delete chat does not delete pending chats in mmsd-tng

Chatty will not delete a sent MMS unless either it is a) sent (delivery reports turned off) or b) delivered (delivery reports turned off).

However, if a user decides to delete the chat before these events, the pending MMS will reappear upon a sending/delivery update, which will confuse the user.

The function to delete a chat should have a call to chatty-mmsd to tell mmsd-tng to delete all MMSes related to that chat, that way MMSes will not reappear.

This is part of the issue in #606 (closed)