[mlir] AsyncRuntime: disable threading until test flakiness is fixed

ExecutionEngine/LLJIT do not run globals destructors in loaded dynamic libraries when destroyed, and threads managed by ThreadPool can race with program termination, and it leads to segfaults.

TODO: Re-enable threading after fixing a problem with destructors, or removing static globals from dynamic library.

Differential Revision: https://reviews.llvm.org/D92368