subtitles: Fix double text with libass rendered plaintext subs

subtitles: Fix double text with libass rendered plaintext subs

When using libass to render plaintext (non-SSA/ASS) subtitles the code
in update_subtitles() still called set_osd_subtitle() in one case,
causing the global vo_sub variable containing non-libass subtitles to
be set. Under some circumstances this resulted in both a
libass-rendered and non-libass-rendered version of the same subtitle
appearing on screen. Fix by running the subtitle clearing code (which
called set_osd_subtitle) only when libass is not used.