Revert leak-fix part from commit 6cac5a9da

I thought it leaks there, but the 'mailbox' variable is freed elsewhere,
thus the added code caused a double-free.
[IMAPx] Avoid useless console warnings after a call to g_rename()

Some cases when g_rename() is called with nonexistent source file
are valid, like when appending a message to a (Sent) folder, thus
do not claim on console in such situations.
Correct a typo in CalDAV's Makefile.am