prioritisetransaction RPC had this argument removed in PR9602, then re-added in
PR10252, then re-named in PR10488.
Since the intermediate states are undesirable and I already did the renaming
to 'dummy' in D4812 (landed), this pulls together the overall remaining effects
of the PRs which are ultimately small:
- modify documentation slightly and force argument to be 0 instead of ignoring.
- change RPC tests to not pass priority, and use keyword arguments instead.
- simplify mining_prioritisetransaction test slightly (from 9602).
The release note is taken verbatim from Core's 0.15 release notes.