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.
validation: Make VerifyDB level 4 interruptible
This is a backport of core#19142