ProjectExplorer: Make BuildStep::immutable use a data member

Default to false, remove all no-op reimplementations.

And rename the getter to isImmutable according to the rules.

Change-Id: I8cce79d88fb59badfa1cffcf30a46f7ff3b09e8b
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/android/androiddeployqtstep.h

@@ -97,7 +97,6 @@ private:
     void slotSetSerialNumber(const QString &serialNumber);
 
     ProjectExplorer::BuildStepConfigWidget *createConfigWidget() override;
-    bool immutable() const override { return true; }
 
     void processReadyReadStdOutput(DeployErrorCode &errorCode);
     void stdOutput(const QString &line);