telepathyClient: Always clear pending messages on destroy

telepathyClient: Always clear pending messages on destroy

Since commit 82950ecea, we acknowledge pending messages when closing a
chat notification for a channel we are handling to prevent the channel
from popping up again immediately. While this isn't an issue for channels
we don't handle, the unread messages of the destroyed notification are
still considered for the messages indicator in the top bar, which is
clearly confusing (in particular when we end up showing the indicator
without any notifications in the list). As it's arguably correct to not
meddle with a channel handled by someone else, just reset the cache of
pending messages to address this issue.

https://bugzilla.gnome.org/show_bug.cgi?id=770888