Remove last Continuation in list when cancelled

[rfc2822]

          What happens in the case the the cancelled Continuation was the last one in the list?

Then pendingDecisions[cert] will forever remain an empty list, right? That would also mean that the UI (getUserDecision()) will not be called again.

This may even be related to bitfireAT/davx5-ose#861

(I think the whole cert4android architecture needs more attention, maybe after this PR, which is hopefully the first step)

Originally posted by @rfc2822 in #50 (comment)

[rfc2822]

This is about https://github.com/bitfireAT/cert4android/pull/50/files/12f1b4596fd26750094877df8be908c4930bf61e#r1991806967