Merge #10191: [trivial] Rename unused RPC arguments 'dummy'

Summary:
0ef7de9 [RPCs] Remove submitblock parameters argument help text (John Newbery)

Tree-SHA512: f39ad4bb3006e3d722fa51ae2ab4a48726e740993d6bed5737b355d1e0a99cf475ca9519f97adf8cde1b7187b14a24d5951ce4f34624d01d9ef84b49124c2894

Backport of Core PR10191
https://github.com/bitcoin/bitcoin/pull/10191/

Test Plan:

make check
test_runner.py

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3599