DOC: remove unused files, they cause a warning in Sphinx

Documentation/Cookbook/rst/Installation_Linux_xdk.rst

-Synopsis
---------
-
-This archive provides a development kit for the ORFEO ToolBox library which
-allows to easily compile OTB and facilitate the creation of programs based on
-OTB.
-
-It contains shared libraries of OTB dependencies and helper scripts for the
-configuration and the compilation of the library.
-
-Note that OTB sources are not included in the archive (see instructions below).
-
-Prerequisites
--------------
-
--  cmake (>= 3.2)
--  gcc (>= 4.9)
--  git
-
-Extract XDK
------------
-
-::
-
-    cd /opt (You can use another a directory of your own choice)
-    chmod +x OTB-|release|-xdk-Linux64.run
-    ./OTB-|release|-xdk-Linux64.run
-
-**You now will have a directory named OTB-|release|-xdk-Linux64 in \'/opt\'**
-
-Setup environment
------------------
-There is a helper script in package called **xdkenv.profile**. This script export
-required environments to configure and build OTB.
-
-::
-
-    cd /opt/OTB-|release|-xdk-Linux64
-    . ./xdkenv.profile
-
-Test environment setup by checking value of CMAKE_PREFIX_PATH
-
-::
-
-    echo $CMAKE_PREFIX_PATH
-
-The above command will output:
- /opt/OTB-|release|-xdk-Linux64
- 
-
-Download, Configure and build OTB
----------------------------------
-
-::
-
-    mkdir -p /opt/OTB/build && cd /opt/OTB
-    git clone --depth=1 --branch=develop https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb.git source
-    cd build && cmake ../source 
-    make -j2
-
-
-Notes
-~~~~~
-
-* By default, all OTB\_USE\_\* variables in OTB are set to FALSE except for 6S and SiftFast. You can check/activate them in your cmake-gui/ccmake.
-
-* If you need to activate Qt4 modules (Monteverdi, Ice, QtWidget, ApplicationEngine ..), you must set *QT_BINARY_DIR*.
-
-::
-
-    cd /opt/OTB/build && cmake -DOTB_USE_QT4=TRUE -DQT_BINARY_DIR=$CMAKE_PREFIX_PATH/bin
-
-This is because Qt tools ( moc, rcc, et al) are reported as not found by cmake due to incorrect path in "qmake -query"
-
-

Documentation/Cookbook/rst/Installation_Macos_xsk.rst

-README.md
-

Documentation/Cookbook/rst/Installation_Windows_xdk.rst

-README.md