Fix undefined behaviour errors due to long int overflow in Amount

Summary:
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.

Test Plan:
Build with the undefined sanitizer enabled then

./src/test/test_bitcoin

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2179