After 79fd1cb2c6 the methods for running
QtConcurrent algorithms in the blocking mode aren't used anymore. Since
ThreadEngineBase and ThreadEngineStarter classes aren't meant to be used
externally, it should be fine to remove startBlocking() methods now.
Removed the unused code and adjusted the tests accordingly.
Change-Id: Ifb13820ce207869d6f720bcb5be8d35bb355fe33
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>