doc: Clean up the build.xml Ant files

doc: Clean up the build.xml Ant files

* Remove the "download" targets. We do not store the doc on a
  wiki anymore.
* Remove the pre-Luna Mylyn task location
* Keep only one instance of some commonly-used files, directly
  under the doc/ directory.

Change-Id: Ibfad6aa2ba332ebb3aec2de255a93fea38d5b1f7
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/41031
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>