qapi: Stop enforcing "type name should not end in 'Kind'

I'm about to convert simple unions to flat unions, then drop simple
union support.  The conversion involves making the implict enum types
explicit.  To reduce churn, I'd like to name them exactly like the
implicit types they replace.  However, these names are reserved for
the generator's use.  They won't be once simple unions are gone.  Stop
enforcing this naming rule now rather than then.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-3-armbru@redhat.com>