videoio: remove cap_libv4l in favour of cap_v4l

cap_libv4l depends on an external library (libv4l) yet is still larger
(1966 loc vs 1822 loc).
It was initially introduced copy pasting cap_v4l in order to offload
various color conversions to libv4l.
However nowadays we handle most of the needed color conversions inside
OpenCV. Our own implementation is better tested and (probably) also
better performing. (as it can optionally leverage IPP/ OpenCL)

Currently cap_v4l is better maintained and generally the code is in
better shape. There is however an API
difference in getting unconverted frames:
* on cap_libv4l one need to set `CV_CAP_MODE_GRAY=1` or
`CV_CAP_MODE_YUYV=1`
* on cap_v4l one needs to set `CV_CAP_PROP_CONVERT_RGB=0`

the latter is more flexible though as it also allows accessing undecoded
JPEG images.

fixes #4563

modules/videoio/include/opencv2/videoio.hpp

@@ -89,7 +89,7 @@ See @ref videoio_overview for more information.
 enum VideoCaptureAPIs {
        CAP_ANY          = 0,            //!< Auto detect == 0
        CAP_VFW          = 200,          //!< Video For Windows (obsolete, removed)
-       CAP_V4L          = 200,          //!< V4L/V4L2 capturing support via libv4l
+       CAP_V4L          = 200,          //!< V4L/V4L2 capturing support
        CAP_V4L2         = CAP_V4L,      //!< Same as CAP_V4L
        CAP_FIREWIRE     = 300,          //!< IEEE 1394 drivers
        CAP_FIREWARE     = CAP_FIREWIRE, //!< Same value as CAP_FIREWIRE