BGR555 has never been working as alpha supporting format. Remove the false setting.

Originally committed as revision 7105 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.c

@@ -269,7 +269,7 @@ static const PixFmtInfo pix_fmt_info[PIX_FMT_NB] = {
     },
     [PIX_FMT_BGR555] = {
         .name = "bgr555",
-        .nb_channels = 4, .is_alpha = 1,
+        .nb_channels = 3,
         .color_type = FF_COLOR_RGB,
         .pixel_type = FF_PIXEL_PACKED,
         .depth = 5,