Remove undocumented pix_fmt breaking new builds

[Pleune]

This no longer exists in ffmpeg trunk. I see that is exists in older versions, such as 2.7. However, even in those versions, it is not documented.

There are similarly named and now document formats such as AV_PIX_FMT_XV30 and AV_PIX_FMT_XV60, which should probably be supported instead, but I will not add these in this PR.

This solves include/usb_cam/formats/av_pixel_format_helper.hpp:747:32: error: ‘AV_PIX_FMT_XVMC’ was not declared in this scope; did you mean ‘AV_PIX_FMT_XV30’? in my build.