Remove blockprioritypercentage config parameter.

Summary: It's not tested and presumably not used at all. Nobody maintains it.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: markblundeberg, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4778