mempool: remove unused magic number from consistency check

Summary:
Unexplained magic numbers are no good. Since the exact number does not matter, opt for a constant that is less peculiar.

Backport of Bitcoin Core PR16056
https://github.com/bitcoin/bitcoin/pull/16056

Test Plan:

ninja check

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4278