avformat/img2enc: remove webp extension as we have a dedicated webp muxer

Found-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/img2enc.c

@@ -194,7 +194,7 @@ AVOutputFormat ff_image2_muxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("image2 sequence"),
     .extensions     = "bmp,dpx,jls,jpeg,jpg,ljpg,pam,pbm,pcx,pgm,pgmyuv,png,"
                       "ppm,sgi,tga,tif,tiff,jp2,j2c,j2k,xwd,sun,ras,rs,im1,im8,im24,"
-                      "sunras,webp,xbm,xface,pix,y",
+                      "sunras,xbm,xface,pix,y",
     .priv_data_size = sizeof(VideoMuxData),
     .video_codec    = AV_CODEC_ID_MJPEG,
     .write_header   = write_header,