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.
Merge topic 'vs-no-indirect-output' into next

806e623 VS: Avoid leaking child process output back to IDE (#14266)