rpc: Remove unused boost::this_thread::interruption_point

Summary:

There are predefined interruption points for boost::thread: https://www.boost.org/doc/libs/1_71_0/doc/html/thread/thread_management.html#interruption_points

However, the rpc threads are std::thread, which does not have an std::thread::interrupt member function to request interruption: https://dev.visucore.com/bitcoin/doxygen/httpserver_8cpp.html#ae1a63374e18b9abd348eb74e4243ea34

Thus, the interruption points can be removed.

This is a backport of Core PR17382

Test Plan: ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8256