Don't specify the C++ standard explicitely for Cppcheck

Summary:
Cppcheck defaults the most recent standard by default, and this will
prevent from failing if the very latest cppcheck version is not used.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8517