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>
Remove obsolete dev-docs target from toplevel Makefile

The actual target was removed with commit
5806093a20e20209d4b88ab866ae1e6017c95de7 ("docs: drop doxygen stuff")

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>