Remove the now redundant deleting of the ParseProjectJob when its
project is destroyed. ParseProjectJob objects are created in
ProjectController::reparseProject(), which is called when the project is
already fully open. So the project is guaranteed to be closed before it
is destroyed.

A Project is destroyed via deleteLater() after it is closed, so if the
event loop is busy at the time, &IProject::destroyed can be emitted much
later than IProjectController::projectClosing(). With this commit
ParseProjectJob is killed earlier than it was destroyed without the
commit.

Early-return from ParseProjectJob::queueFilesToParse() not only when the
job has been destroyed, but when it has been killed too. The earlier
return not just avoids unnecessary work, but is essential during the
application shutdown: prevents a crash by not accessing IndexedString
after DUChain::shutdown(). Note that KJob::kill() calls deleteLater(),
so a job can be destroyed a long time after it is killed if the event
loop is busy, as it is at shutdown. ParseProjectJob is killed early in
the shutdown process from RunController::cleanup() - before
ProjectController::cleanup(), which would kill it otherwise, and
(importantly to prevent the crash) before DUChain::shutdown().

Remove the shuttingDown() check from ParseProjectJob::start(), because
this member function doesn't access globals on its own. The appropriate
safety checks are now performed in the scheduled
ParseProjectJob::queueFilesToParse().

Don't call deleteLater() in ParseProjectJob::doKill() just before
returning true: rely on auto-delete KJob base class to call it.

BUG: 427387
FIXED-IN: 5.6.1