Remove deprecated CMake variable

Two CMake variable are marked as deprecated and "#TODO next release:
remove this". It is introduced in https://github.com/opencv/opencv/pull/11167
which is before the release of 4.0. Since it's deprecated almost six
years ago, I think it's OK to remove them.

cmake/OpenCVDetectCXXCompiler.cmake

@@ -28,22 +28,8 @@ if(NOT DEFINED CV_GCC AND CMAKE_CXX_COMPILER_ID MATCHES "GNU")
 endif()
 if(NOT DEFINED CV_CLANG AND CMAKE_CXX_COMPILER_ID MATCHES "Clang")  # Clang or AppleClang (see CMP0025)
   set(CV_CLANG 1)
-  set(CMAKE_COMPILER_IS_CLANGCXX 1)  # TODO next release: remove this
-  set(CMAKE_COMPILER_IS_CLANGCC 1)   # TODO next release: remove this
 endif()
 
-function(access_CMAKE_COMPILER_IS_CLANGCXX)
-  if(NOT OPENCV_SUPPRESS_DEPRECATIONS)
-    message(WARNING "DEPRECATED: CMAKE_COMPILER_IS_CLANGCXX support is deprecated in OpenCV.
-    Consider using:
-    - CV_GCC    # GCC
-    - CV_CLANG  # Clang or AppleClang (see CMP0025)
-")
-  endif()
-endfunction()
-variable_watch(CMAKE_COMPILER_IS_CLANGCXX access_CMAKE_COMPILER_IS_CLANGCXX)
-variable_watch(CMAKE_COMPILER_IS_CLANGCC access_CMAKE_COMPILER_IS_CLANGCXX)
-
 
 # ----------------------------------------------------------------------------
 # Detect Intel ICC compiler