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.cpp

@@ -321,11 +321,6 @@ void CMakeBuildStep::setBuildTarget(const QString &buildTarget)
     emit targetToBuildChanged();
 }
 
-void CMakeBuildStep::clearBuildTargets()
-{
-    m_buildTarget.clear();
-}
-
 QString CMakeBuildStep::toolArguments() const
 {
     return m_toolArguments;