cmLocalUnixMakefileGenerator3: Remove unnecessary local variable

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -1200,9 +1200,8 @@ void cmLocalUnixMakefileGenerator3::AppendEcho(
 }
 
 std::string cmLocalUnixMakefileGenerator3::CreateMakeVariable(
-  const std::string& sin, const std::string& s2)
+  std::string const& s, std::string const& s2)
 {
-  std::string s = sin;
   std::string unmodified = s;
   unmodified += s2;
   // if there is no restriction on the length of make variables