Leave CPACK_PACKAGING_INSTALL_PREFIX's value as per CMake's default.

Close #2166

CMakeLists.txt

@@ -140,9 +140,6 @@ elseif (CPACK_SYSTEM_NAME STREQUAL Linux)
             endif ()
         endif ()
     endif ()
-    # Note to package maintainer: comment out below line to revert the prefix from '/usr/local' (default for CMAKE_INSTALL_PREFIX) to '/usr' (default for CPACK_PACKAGING_INSTALL_PREFIX)
-    # Revisit this later, some users insist it should be '/usr' (see https://github.com/urho3d/Urho3D/issues/2166), even though clearly we are not trying to build "system package" here
-    set (CPACK_PACKAGING_INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX})
 endif ()
 if (URHO3D_64BIT)
     set (CPACK_SYSTEM_NAME ${CPACK_SYSTEM_NAME}-64bit)