app: Clean up retry data of removed accounts

Telepathy's account IDs use a pattern of //num, which is mostly redundant for us: We only support idle/irc, and for predefined networks the original nick (which ends up being the identifier recorded in the ID) is the login username. So usually account IDs only differ in the uniquifying number - starting from 0, telepathy will simply pick the smallest number that hasn't been taken yet. That strategy means that when removing an account, the next account that is created may well use an identical ID. In case we have retry data (like alternative servers) for the old account, we must make sure to not accidentally apply it to the new one, so clean it on account removal.

Fixes #27 (closed)