[SECP256K1] Remove num/gmp support

Summary:

The whole "num" API and its libgmp-based implementation are now unused.
Remove them.

Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/1f233b3fa05eb29a744487e0682d925055fb0d4c

Depends on D9407.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9408