Enable letter-spacing for Firefox HTML export

Kerning (letter-spacing in CSS) was disabled for Firefox HTML
export mode. There is no need to do that since Firefox does
support this feature.

Change-Id: I237063d52f50d79517bf83d5d1904f2164095d31
Reviewed-on: https://gerrit.libreoffice.org/8947
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>