assumptions: Assume C++17

Summary:
Backport of core#21274.
The __cplusplus expected value has already been updated, so only
update the comments and remove the #ifdef.

Test Plan:

ninja

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9319