Fix undefined behaviour errors due to long int overflow in Amount
Running the units tests with the undefined behaviour sanitizer throws
runtime errors: signed integer overflow in file amount.h.
This is due to the INF_PRIORITY constant, which is no longer used in the
software. It can be safely removed.
Build with the undefined sanitizer enabled then
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2179