Merge #10560: Remove unused constants

Summary:
dce8239 Comment out unused constant REJECT_DUST (practicalswift)
3289ef4 Remove unused constant MEMPOOL_GD_VERSION (practicalswift)

Tree-SHA512: 8dd772c963037cd0a1db0d5bf5f7a5cce3f5cd8fb6fa983949f9f0c1c8312ffd251a62c1e5d32f3584d2bcc562d8c8a9bf7900609dee9a4f12898caa2be9c38f

I actually went a head and deleted REJECT_DUST since it wasn't used anywhere and it made no sense to just leave it in even if it was uncommented.

Backport of Core PR10560
https://github.com/bitcoin/bitcoin/pull/10560/files

Test Plan:

make check
test_runner.py

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3260