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/cmakebuildtarget.h

@@ -57,8 +57,6 @@ public:
     QStringList compilerOptions;
     ProjectExplorer::Macros macros;
     QList<Utils::FileName> files;
-
-    void clear();
 };
 
 } // namespace CMakeProjectManager