SmtpJob: Fix use of KCompositeJob

SmtpJob overrides slotResult, but this had two issues. First, it connected
to LoginJob::result itself, though addJob already does that. Second, in two
cases it did not remove finished jobs from the list of subjobs, leading to
dangling pointers and messing up state. This was most likely only reached
in the case of an expired XOAUTH2 access_token and caused the job to fail
after a successful refresh.

BUG: 406839
BUG: 409122
BUG: 421664
BUG: 456923