pixfmt: remove misleading and broken documentation

This was probably broken some time ago. The breakage is now part of the
ABI. For example, we have:

    AV_PIX_FMT_XYZ12BE
    AV_PIX_FMT_NV16
    AV_PIX_FMT_NV20LE

AV_PIX_FMT_NV20LE is wrong. It has the value 113, but as little-endian
format it should be even. This must have been quite obvious when these
formats were added (because of the AV_PIX_FMT_XYZ12BE entry), but
nobody cared or knew about this.

The future libavutil major bump will also break this additionally,
because disabling FF_API_VDPAU will remove an odd number of entries from
the middle of the enum.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavutil/pixfmt.h

@@ -54,11 +54,6 @@
  * For all the 8bit per pixel formats, an RGB32 palette is in data[1] like
  * for pal8. This palette is filled in automatically by the function
  * allocating the picture.
- *
- * @note
- * Make sure that all newly added big-endian formats have pix_fmt & 1 == 1
- * and that all newly added little-endian formats have pix_fmt & 1 == 0.
- * This allows simpler detection of big vs little-endian.
  */
 enum AVPixelFormat {
     AV_PIX_FMT_NONE = -1,