Check*CompilerFlag: Do not set result as a normal variable too

Refactoring in commit cb984c66 (Check*CompilerFlag: Modernize modules,
2019-12-09, v3.17.0-rc1~320^2) accidentally left the result set as a
normal variable in addition to as a cache entry.  This is not specified
by the documentation, and is not the behavior in CMake 3.16 and below.

Fixes: #21207

Modules/CheckCCompilerFlag.cmake

@@ -54,5 +54,4 @@ function(check_c_compiler_flag _flag _var)
   foreach(v IN LISTS _locale_vars)
     set(ENV{${v}} ${_locale_vars_saved_${v}})
   endforeach()
-  set(${_var} "${${_var}}" PARENT_SCOPE)
 endfunction()

Modules/CheckCXXCompilerFlag.cmake

@@ -54,5 +54,4 @@ function(check_cxx_compiler_flag _flag _var)
   foreach(v IN LISTS _locale_vars)
     set(ENV{${v}} ${_locale_vars_saved_${v}})
   endforeach()
-  set(${_var} "${${_var}}" PARENT_SCOPE)
 endfunction()