Remove <!-- --> in <style> in calc export to HTML

Comment was used to hide <style> contents from old browsers.
No longer necessary for browsers now.

Change-Id: I49290b8dc45db173dc6f29d131d5001be2b27b6e
Reviewed-on: https://gerrit.libreoffice.org/8077
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>