Remove dead qemu variables from toplevel Makefile

Remove dead qemu variables from toplevel Makefile

The variables should have been removed already in
7ed06911dc596d23f21dee209b7f2db294e34b56 ("Remove dead qemu code from
toplevel Makefile"), but I did not spot it at this time.
Now it looks like the intention initially was to let the install-tools
depend on tools/qemu-xen-traditional-dir. But that did not work because
there was the typo in the variables (TGT vs. TARGET), and
CONFIG_QEMU_TRAD was not defined anyway.

So remove QEMU_TRAD_DIR_TARGET and QEMU_XEN_DIR_TARGET because they are
not defined at all.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>