build: remove tboot make targets

The tboot targets are woefully out of date. These should really be
retired because setting up tboot is more complex than the build process
for it.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Christopher Clark <christopher.clark6@baesystems.com>

README

@@ -199,8 +199,3 @@ http://sourceforge.net/projects/tboot.  This project hosts the code in a
 mercurial repo at http://tboot.sourceforge.net/hg/tboot.hg and contains
 tarballs of the source.  Instructions in the tboot README describe how
 to modify grub.conf to use tboot to launch Xen.
-
-There are optional targets as part of Xen's top-level makefile that will
-download and build tboot: install-tboot, build-tboot, dist-tboot, clean-tboot.
-These will download the latest tar file from the SourceForge site using wget,
-then build/install/dist according to Xen's settings.