[qa] [nit] remove extranous variable in mining_prioritisetransaction.py

Summary: As per title

Test Plan:

./test/functional/test_runner.py mining_prioritisetransaction.py

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

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2854