VS: Avoid leaking child process output back to IDE (#14266)

The VS IDE sets the environment variable VS_UNICODE_OUTPUT when
executing build rules in order to tell MS tools to report output through
a back door instead of through stdout/stderr.  Unset this variable so
that CMake can capture or properly redirect all output from processes it
runs even when running inside a VS IDE build environment.

This generalizes the special cases fixed by commit 80d045b0 (When
GetPrerequisites.cmake runs dumpbin while running inside the VS IDE...,
2008-05-01) and commit 44aff73d (ExternalProject: Avoid bleed-through
output when logging, 2011-01-06), so drop special handling of
VS_UNICODE_OUTPUT in those instances.

Modules/ExternalProject.cmake

@@ -961,7 +961,7 @@ endif()
         set(sep ";")
       endif()
     endforeach()
-    set(code "set(ENV{VS_UNICODE_OUTPUT} \"\")\n${code}set(command \"${cmd}\")${code_execute_process}")
+    set(code "${code}set(command \"${cmd}\")${code_execute_process}")
     file(WRITE ${stamp_dir}/${name}-${step}-impl.cmake "${code}")
     set(command ${CMAKE_COMMAND} "-Dmake=\${make}" "-Dconfig=\${config}" -P ${stamp_dir}/${name}-${step}-impl.cmake)
   endif()
@@ -971,7 +971,6 @@ endif()
   set(logbase ${stamp_dir}/${name}-${step})
   file(WRITE ${script} "
 ${code_cygpath_make}
-set(ENV{VS_UNICODE_OUTPUT} \"\")
 set(command \"${command}\")
 execute_process(
   COMMAND \${command}