add missing lock assertion to removeForReorg

Summary:
removeForReorg has the EXCLUSIVE_LOCKS_REQUIRED(cs, cs_main) annotation.
It calls TestLockPointValidity which also has the cs_main lock annotation and assertion.

This lock assertion would normally be added in commit https://github.com/bitcoin/bitcoin/pull/22677/commits/bb9078ed51159fa162484f16993313ed6cf980e3, but it is useful immediately.

Test Plan:
with clang and DEBUG:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12442