Remove ::dustRelayFee global

Summary:
This concludes backport of core#25648
https://github.com/bitcoin/bitcoin/pull/25648/commits/fadc14e4f514e7167723285e0ac3d4a7149bbee6

The paymentserver test now needs a proper node context to access getDustRelayFee. This was the last remaining callsite in the codebase that used MakeNode() without passing a NodeContext, so we can remove the error-prone nullptr default value.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15756