[64-bit ints] 32-bit integers? I never knew you

Summary:
After this upgrade, it's safe to pretend we never had 32-bit integers: depart from me.

This keeps the node codebase clean, and we pretend like we always had 63+sign-bit integers.

We don't update agora's defaults etc., which can be done in a separate diff.

Also, feature_63_bit_ints is removed because this is a test just for the activation, which ostensibly worked very well.

Test Plan: ninja check

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: Fabien, #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18955