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

doc/py_tutorials/py_setup/py_setup_in_fedora/py_setup_in_fedora.markdown

@@ -62,12 +62,11 @@ yum install cmake
 yum install python-devel numpy
 yum install gcc gcc-c++
 @endcode
-Next we need **GTK** support for GUI features, Camera support (libdc1394, libv4l), Media Support
+Next we need **GTK** support for GUI features, Camera support (libdc1394, v4l), Media Support
 (ffmpeg, gstreamer) etc.
 @code{.sh}
 yum install gtk2-devel
 yum install libdc1394-devel
-yum install libv4l-devel
 yum install ffmpeg-devel
 yum install gstreamer-plugins-base-devel
 @endcode

doc/py_tutorials/py_setup/py_setup_in_ubuntu/py_setup_in_ubuntu.markdown

@@ -66,12 +66,11 @@ sudo apt-get install python-devel numpy
 sudo apt-get install gcc gcc-c++
 ```
 
-Next we need **GTK** support for GUI features, Camera support (libv4l), Media Support
+Next we need **GTK** support for GUI features, Camera support (v4l), Media Support
 (ffmpeg, gstreamer) etc.
 
 ```
 sudo apt-get install gtk2-devel
-sudo apt-get install libv4l-devel
 sudo apt-get install ffmpeg-devel
 sudo apt-get install gstreamer-plugins-base-devel
 ```