[SECP256K1] Remove unused scalar_sqr
Summary:
Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/5437e7bdfbffddf69fdf7b4af7e997c78f5dafbf
Depends on D9406.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9407