Remove XLA Client::ExecuteAsync API.

This is a simplification, but also: The ExecuteAsync makes it easy to do
the wrong thing.

ExecuteAsync lets you easily start a computation and then infeed/outfeed
to it, all without starting any threads yourself.

But in practice, if you're using infeed or outfeed, you're probably
using both.  For good performance, you should overlap infeeds and
outfeeds, which means you need to run them on separate threads.  But
then you might as well use a thread for Execute, as well.

PiperOrigin-RevId: 163509892