Fix a crash when using the blocking API of ParsingRunnerManager

Both QThreadPool and ParsingRunnerManager were trying to delete the
same class instance. Now ParsingRunnerManager only keeps track of
the number of active parsers, but does not attempt to delete them
once they finish.

A simpler fix would have been to call setAutoDelete(false) in the
ParsingTask ctor, but this approach seems cleaner.

CCBUG: 357157