[SECP256K1] Remove unused secp256k1_fe_inv_all_var

Summary:

This function is only used in tests, since its functionality was inlined
into secp256k1_ge_set_all_gej_var in #553.

Backport of secp256k1#878

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9384