[sigcheck] Remove redundant sigcheck in CheckInputs

Summary: The call to check checks for sigops - which is a better place to do it anyways.

Test Plan: Updated test to cover the error.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5298