[SECP256K1] Rip out non-endomorphism code

Summary:
Partial backport 8/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/4232e5b7da0a68adc14fa4b481f7e106403c200d

Updates are necessary to accomodate our build system and Travis
variations.

Depends on D8043.

Test Plan:

ninja check-secp256k1
make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8044