Remove op== on PSBTs; check compatibility in Merge
Summary:
Remove the op== on PartiallySignedTransaction, which only checks that the CTransactions are equal. Instead, check this directly in Merge, and return false if the CTransactions are not equal (so the PSBTs cannot be merged.)
Partial backport of core PR14978:
https://github.com/bitcoin/bitcoin/pull/14978/commits/78b9893d020e8b1351565f1adbf591cb32f6dc90
Test Plan:
ninja all check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5738