ts-xen-install: Drop obsolete --check and $checkmode

ts-xen-install: Drop obsolete --check and $checkmode

Since 5edec3222ff14b8200d5b41ed9af3c8d04c27314
  ts-xen-build-check: reimplementation ...
in 2011 we have a fully general version, and the --check option
is no longer used.

And since 467d6239daa352483ec8fef003bd4fc723702b77
  pygrub: guest kernel install: move target_extract_jobdist_path into Osstest
also in 2011, --check has not worked properly, because that
commit moved the relevant code somewhere where $checkmode was
not available and simply dropped the check.

So remove the vestigial code in ts-xen-build.  As for @ARGV, we simply
die if we see something that looks like an option.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>