Update /doc/bips.md from Core to ABC

Summary:
Remove BIP 125 (RBF)
Remove BIP 144, 144, 145 (Segwit)
Leave BIP 143 in place, used in Bitcoin ABC replay protected sighash

Test Plan: None

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D316