imgconvert: remove PixFmtInfo.pixel_type field

The field is only used for checking if a format is YUV planar
and if the format is paletted.

This patch replaces the current is_planar_yuv() code with a function
which uses the pixdesc information. The new implementation is less
efficient, but this should not be an issue as the functions which use
it (av_picture_crop/pad()) are deprecated and currently never used in
the codebase.
