writerfilter: RTF import: ignore content of \upr, except for \ud

The content of \upr and its nested \ud destination should be the same
modulo text encoding, so ignore the legacy one.

Change-Id: I007151e3075a9ac879a486ec3b5fb8549435dbba
(cherry picked from commit b46956e73b6daff2578110c0395fd80150dc0b0b)
Reviewed-on: https://gerrit.libreoffice.org/10699
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>