cmake/cxx11.cmake: dropped explicit setting of C++17 for Visual Studio with Qt6 (danmar#7464)

this is not necessary since CMake is setting C++17 for the GUI files
(regardless of the compiler)

Author: firewave

cmake/cxx11.cmake

@@ -1,9 +1,5 @@
 macro(use_cxx11)
-  # some GitHub Action Windows runners randomly fail with a complaint that Qt6 requires a C++17 compiler
-  if(MSVC AND USE_QT6)
-    # CMAKE_CXX_STANDARD 17 was added in CMake 3.8
-    set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to use")
-  elseif(USE_BOOST AND USE_BOOST_INT128)
+  if(USE_BOOST AND USE_BOOST_INT128)
     # Boost.Math requires C++14
     set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to use")
   else()