[validation] Remove comment about AcceptToMemoryPool()

Summary:

"This logic is not necessary for memory pool transactions, as
AcceptToMemoryPool already refuses previously-known transaction ids
entirely." refers to the logic at
https://github.com/bitcoin/bitcoin/blob/a206b0ea12eb4606b93323268fc81a4f1f952531/src/main.cpp#L484-L486,
which was later removed in commit rABC450cbb0944cd20a06ce806e6679a1f4c83c50db2.

This is a backport of core#23173
https://github.com/bitcoin/bitcoin/pull/23173/commits/4c24142b1ec121623f81ba644d77341bc1bd88dd

Test Plan: NA

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12233