ProjectExplorer: Replace virtual BuildStepConfigWidget::showWidget

... by a bool member.

One reason less for the need of custom derived classes, i.e. one
step forward to a setup where the then-generic config widget can be
created in BuildStepListWidget::init() and filled with aspect-
created subwidget, reducing the need of per-BuildStep boilerplate
similar to what was done in RunConfiguration recently.

Task-number: QTCREATORBUG-19985
Change-Id: I85d26bdb6b35d0d6715782214328a40eef87286e
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/android/androidpackageinstallationstep.h

@@ -61,7 +61,6 @@ public:
 
     QString summaryText() const override;
     QString displayName() const override;
-    bool showWidget() const override;
 private:
     AndroidPackageInstallationStep *m_step;
 };