VTK_LEGACY_REMOVE should default to OFF

This looks like something that slipped through in the modularization
commit, turning it off by default so that typical builds contain legacy
removal warnings as intended.

Change-Id: I678a7156bb70a8c1adad463b49a2a23db76d569a
Merge topic 'legacy-remove-default' into master

cc8d4251 VTK_LEGACY_REMOVE should default to OFF