Make max tx input script sig size 1650 a constant.

Summary: Lot of comments for it in .cpp file, make it constant would be better.

Test Plan: make check

Reviewers: #bitcoin_abc, O1 Bitcoin ABC, jasonbcox

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1648