cleanup more unused code after removal of chained-tx logic (TestLockPointValidity)

Summary:
TestLockPointValidity is unused since D13133. Remove it, and remove include for GetConsensus that is no longer used.

LockPoint::maxInputBlock is no longer used by anything, TestLockPointValidity was the only user. We no longer need to compute it in validation and we can remove it from the struct.
As a result we longer need to include chain.h (CBlockIndex).

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15788