remove unused CScript::IsCommitment

Summary: This was introduced in D147 to be used by an opt-in anti replay feature. The feature was sunset at block 530000 and removed from the codebase in D1412. Since then, the only callsite is a unit test.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15036