Remove obsolete comment from MANDATORY_SCRIPT_VERIFICATION_FLAGS

Summary: More than just P2SH now, so comment is no longer accurate. Fix this.

Test Plan: Read it.

Reviewers: deadalnix, markblundeberg, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D2921