SL-18837: Remove C++17 override, given build-variables change.

Until now, the viewer has explicitly set(CMAKE_CXX_STANDARD 17) with a comment
explaining that we hadn't dared add -std=c++17 to build-variables/variables
because we didn't know if we could build all the component autobuild packages
with that switch. Now that we've successfully built all of them with that
switch, we've updated the viewer branch of build-variables, so we can remove
the viewer's CMAKE_CXX_STANDARD override.

indra/CMakeLists.txt

@@ -29,15 +29,6 @@ else()
   set( USE_AUTOBUILD_3P ON )
 endif()
 
-# The viewer code base can now be successfully compiled with -std=c++14. But
-# turning that on in the generic viewer-build-variables/variables file would
-# potentially require tweaking each of our ~50 third-party library builds.
-# Until we decide to set -std=c++14 in viewer-build-variables/variables, set
-# it locally here: we want to at least prevent inadvertently reintroducing
-# viewer code that would fail with C++14.
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-
 include(Variables)
 include(BuildVersion)