Previously, the fieldmarks couldn't be removed with backspace or
deletee when cursor was right/left to them.

After commits f72b866c and
c34fc452, deletion worked wrong
(it didn't delete the mark from mark manager; in case of text
field, it removed one field's boundary).

Now single backspace/delete properly removes the whole fieldmark,
replacing it with its contents if applicable.

Change-Id: Id26e6e4e40e274d9fd6f0224f3e2b4fe33c369b7
Reviewed-on: https://gerrit.libreoffice.org/33812Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>