[tests] remove test_bitcoin.h dependency on txmempool.h

Summary:
It turns out nothing was actually using the LockPoints field of the mempool
entry helper, and (looking to future) nothing in bitcoin Core's tests use
this field either. Getting rid of it lets us speed up test compilation/
recompilation by removing the (rather heavy) txmempool.h dependency and
its various indirect dependencies.

If it's needed again in future, it's easy enough to reintroduce.

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4923