Remove unused apps/dav/js/schedule-response.js

File is not included in the schedule-response-options template and is
commented out. The commented code relies on the buttonset() jQuery
plugin, which is deprecated.
https://api.jqueryui.com/buttonset/



If this code was to be re-introduced, it would take a different form.

Signed-off-by: François Freitag <mail@franek.fr>