Remove IsPayToWitnessScriptHash

Summary:
Function is not used anywhere and seems a leftover from the removal of
Core's segregated witness.

Test Plan: make -j16 check

Reviewers: deadalnix, jasonbcox, schancel, #bitcoin_abc, matiu

Reviewed By: matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1030