validation: Make VerifyDB level 4 interruptible

Summary:
fa1d5800d9c5e33942b76f6667839a818723dee9

validation: Remove unused boost interruption_point

ActivateBestChain (ABC) is only called in the "msghand" or one of the
RPC threads, neither of which is a boost::thread. However, ABC is also
called in ThreadImport (which currently happens to be a boost::thread).
In all cases, the interruption_point is redundant with the breakpoint in
ABC that triggers when ShutdownRequested()

VerifyDB is only called in the main thread ("init") or one of the RPC
threads, neither of which is a boost::thread.

fa3b4f9b8e54ec07aeb2e5e2333da3e784f7be12

validation: Make VerifyDB level 4 interruptible

This is a backport of core#19142

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9269