These constants were added "speculatively" in PR 10820, almost four years ago, but have never actually been used. We already have issue 10982 that tracks *potentially* extending support for the affected annotation-format, however until that happens I really don't think that we should keep shipping completely unused code in the PDF.js library.
For the MOZCENTRAL build-target, i.e. the Firefox PDF Viewer, this reduces the total bundle size by 1.1 kilo-byte.