Remove another orphan expected-result file.

Remove another orphan expected-result file.

aborted-keyrevoke_2.out was apparently needed when it was added (in
commit 0ac5ad513) to handle the case of serializable transaction mode.
However, the output in serializable mode actually matches the regular
aborted-keyrevoke.out file, and AFAICT has done so for a long time.
There's no need to keep dragging this variant along.