tests/qapi-schema: Delete two redundant tests

tests/qapi-schema: Delete two redundant tests

Tests duplicate-key and double-data test the same thing.  The former
predates the latter, and it has a better name.  Delete the latter, and
tweak the former's comment.

Tests include-format-err and include-extra-junk test the same thing.
The former predates the latter, but the latter has a better name and a
comment.  Delete the former.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>