CMake: Remove CMakeBuildTarget::clear()

One of the few users of the non-const FileName::clear().

Change-Id: Ic1fa5c5ec24ff41170317bf46ed61543a6bfcb42
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakecbpparser.cpp

@@ -234,7 +234,7 @@ void CMakeCbpParser::parseBuild()
 
 void CMakeCbpParser::parseBuildTarget()
 {
-    m_buildTarget.clear();
+    m_buildTarget = CMakeBuildTarget();
 
     if (attributes().hasAttribute("title"))
         m_buildTarget.title = attributes().value("title").toString();