qapi: Remove outdated tests related to QMP/branch collisions

Now that branches are in a separate C namespace, we can remove
the restrictions in the parser that claim a branch name would
collide with QMP, and delete the negative tests that are no
longer problematic.  A separate patch can then add positive
tests to qapi-schema-test to test that any corner cases will
compile correctly.

This reverts the scripts/qapi.py portion of commit 7b2a5c2,
now that the assertions that it plugged are no longer possible.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-15-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
