Remove unused CMAKE_BACKWARDS_COMPATIBILITY mark

CMake itself has no calls to cmake_minimum_required with VERSION < 2.6
so this cache variable does not appear at all.

CMakeLists.txt

@@ -19,8 +19,6 @@ IF(CMAKE_BOOTSTRAP)
   UNSET(CMAKE_BOOTSTRAP CACHE)
 ENDIF()
 
-MARK_AS_ADVANCED(CMAKE_BACKWARDS_COMPATIBILITY)
-
 SET(CMake_BIN_DIR ${CMake_BINARY_DIR}/bin)
 
 IF("${CMake_SOURCE_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}")