CMake: Remove clearBuildTarget from CMakeBuildStep

That method makes little sense, considering that there can only ever
be one build target.

Change-Id: I53d555fa7d713dd8c05b4a9f5944908afeee859f
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.h

@@ -64,7 +64,6 @@ public:
     QString buildTarget() const;
     bool buildsBuildTarget(const QString &target) const;
     void setBuildTarget(const QString &target);
-    void clearBuildTargets();
 
     QString toolArguments() const;
     void setToolArguments(const QString &list);