Replace accesses to `sched_active_thread`, `sched_active_pid`, and
`sched_threads` with `thread_get_active()`, `thread_get_active_pid()`, and
`thread_get_unchecked()` where sensible.
This commit removes the #error from the thread_flags header.
This #error makes the usage of
if(IS_USED(MODULE_THAT_DEPENDS_ON_THREAD_FLAGS)) pattern harder,
because the error is triggered each time the header is included.
If a module uses any thread_flags function it will fail in link time
anyway.
