cmSystemTools: Drop CollapseCombinedPath in favor of CollapseFullPath
CollapseCombinedPath function was added by commit 551d3343, but the existing
CollapseFullPath should have been used. Since then a few other uses of
CollapseCombinedPath have appeared. It both duplicates code and is less robust (e.g. #19049 (closed)). We should replace remaining uses of
CollapseCombinedPath and then remove it.