ProjectExplorer: Simplify BuildStepFactory

De-QObject-ify class, tr contexts are now the product's
context, which was the case in some BuildStepFactories already,
and is current state in {Run,DeployConfiguration}Factory.
One spurious object name removed.

De-virtualize canHandle(), it was never overloaded and is not
intended to be overloaded anymore.

Remove unused clone() function.

Change-Id: Iff7fba5f707505f868f94458084a18650535fa23
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -295,7 +295,7 @@ QbsCleanStepFactory::QbsCleanStepFactory()
     registerStep<QbsCleanStep>(Constants::QBS_CLEANSTEP_ID);
     setSupportedStepList(ProjectExplorer::Constants::BUILDSTEPS_CLEAN);
     setSupportedConfiguration(Constants::QBS_BC_ID);
-    setDisplayName(tr("Qbs Clean"));
+    setDisplayName(QbsCleanStep::tr("Qbs Clean"));
 }
 
 } // namespace Internal