ProcessOrphanTx: remove useless setMisbehaving set

Summary:
Original commit:

This starts empty, and is only added to if we're about to
exit the function (so we never read from it).

In the Bitcoin ABC codebase, setMisbehaving was replaced with rejectCountPerNode in D2676, and in D6683 it stopped being used.

This is a backport of core#19498 [3/5]
https://github.com/bitcoin/bitcoin/pull/19498/commits/4763b51bca86fb9e49175619a47cdbef34feaf99

Depends on D10413

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10414