libvlc: Remove unsubstituted @LIBS_libvlc@ from libvlc.pc.in

Since 7e916f4c, @LIBS_libvlc@ no
longer gets substituted in the pkg-config files (as

    VLC_ADD_LIBS([libvlc vlc],[...])

was responsible for invoking AC_SUBST() for the LIBS_libvlc variable.

This fixes building e.g. npapi-vlc after this commit.