[nit] Remove redundant parameter from CTxMemPool::PrioritiseTransaction

Summary:
As per title. Remove extra string parameter that prematurely optimizes the RPC API.
This parameter is an opportunity to pass in a string which doesn't match the uint256
provided.

Also happens to be identical to Core PR 9801.

Test Plan:

make check

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

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, Fabien

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2853