configure: Drop weak dependencies on external libraries for webm muxer

Weak dependencies on external libraries do not obviate having to
explicitly enable these libraries, so the weak dependency does not
simplify the configure command line nor have any real effect.

configure

@@ -2376,7 +2376,6 @@ w64_demuxer_select="wav_demuxer"
 wav_demuxer_select="riffdec"
 wav_muxer_select="riffenc"
 webm_muxer_select="iso_media riffenc"
-webm_muxer_suggest="libopus_encoder libvorbis_encoder libvpx_vp8_encoder libvpx_vp9_encoder"
 wtv_demuxer_select="mpegts_demuxer riffdec"
 xmv_demuxer_select="riffdec"
 xwma_demuxer_select="riffdec"