sanitycheck: Remove pointless shutdown() from BoundedExecutor

Just calls through to concurrent.futures.Executor.shutdown() in the base
class, which has the same signature. Removing it means the base class
version will get used directly.

Fixes this pylint warning:

    W0235: Useless super delegation in method 'shutdown'
    (useless-super-delegation)

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>