ProcessOrphanTx: remove useless done variable

Summary:

There is a keyword that allows us to break out of loops. Use it.

There's a small change in behaviour here: if we process multiple orphans
that are still orphans, then we'll only call mempool.check() once at the
end, instead of after processing each tx.

This is a backport of core#19498 [2/5]
https://github.com/bitcoin/bitcoin/pull/19498/commits/55c79a9cefb6c83cdebbf6c538c471607695b457

Depends on D10412

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10413