Move licenses from /usr/LICENSES to /usr/share/licenses/arrayfire.

Remove /usr/etc/arrayfire_version.txt which, according to the following
comment from CMakeModules/CPackConfig.cmake, is used only to have a
file inside the Debian arrayfire meta package:

    # Create a arrayfire component so that Debian package has a top level
    # package that installs all the backends. This package needs to have
    # some files associated with it so that it doesn't get deleted by
    # APT after its installed.

I couldn't find any other references to this file.