common/async: remove null_yield support for spawn_throttle

common/async: remove null_yield support for spawn_throttle

null_yield support was provided by sync_spawn_throttle_impl, which
itself stores the boost::asio::io_context that it runs on. however,
valgrind reports issues with object lifetimes

each spawn_throttle_handler holds a reference count on the
spawn_throttle_impl to prevent it from going away while coroutines are
in flight. on completion, this spawn_throttle_handler gets invoked on
its associated executor. this invocation may drop the last reference
to spawn_throttle_impl while that associated executor is still trying
to use the io_context that was destroyed with sync_spawn_throttle_impl

finding no good way for a custom executor to preserve the lifetime of
the spawn_throttle_impl, we instead remove support for this flawed
design. the null_yield callers must first spawn a parent coroutine
before using spawn_throttle

Fixes: https://tracker.ceph.com/issues/70965
Signed-off-by: Casey Bodley <cbodley@redhat.com>