oe/utils: remove the ThreadedPool

The ThreadedPool in OE-core is mainly because python2
doesn't have threaded pools but python2 is dead for
some time now and python3 have a ThreadPoolExecutor.

The only local in OE-core where this ThreadedPool
is in use is on the sstate.bbclass that is ported
to the python3 ThreadPoolExecutor.

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>