Delete gej_is_valid_var: unused outside tests

Summary:
This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/d7f39ae4b67ea1ac6f085e6262a5f53afc0c5a25

Depends on D7656

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7657