Remove `BOOST_LEAF_NO_THREADS` from defines

[kammce]

This macro prevents libhal from using an RTOS as it prevents TLS (thread local storage) from being available with Boost.LEAF. With its removal, some other method of determining WHO defines this or other TLS macros in the libhal architecture.

[kammce]

Figuring out how to support non-threaded and threaded applications with libel is critical. Not figuring this out can prevent one or the other from being able to work. And it is unknown if the threaded and unthreaded ABIs can play nicely with each other.