In 161908157d we moved all warnings
coming out of the library folder to /W0 as many of them do not follow
our code-style nor can we force them to.
When i made this change, i took `/external:templates-` to mean
"and that goes for you too, templates" and it decisively does the
opposite leading to /W3 warnings coming out of openvdb
This change removes the flag as it should have never have been added
in the first place.
4cc94679dd