Remove leftover from the schnorr experiment

Summary: Removed the residual file from the schnorr experiment: src/bench_schnorr_verify.c

Test Plan:
make check
python3 qa/pull-tests/rpc-tests.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, MattBitprim

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix, freetrader

Differential Revision: https://reviews.bitcoinabc.org/D247