ProcessOrphanTx: Remove outdated comment

Summary:
Also rename orphan_state to state. Both the comment and the variable
name are leftover from when this logic was part of ProcessMessage().

This is a backport of core#19498 [4/5]
https://github.com/bitcoin/bitcoin/pull/19498/commits/e07c5d94231cefb748f9534ab8ff0b3e2b04c4d8

Depends on D10414

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10415