qapi/source: Remove line number from QAPISourceInfo initializer

With the QAPISourceInfo(None, None, None) construct gone, there's no
longer any reason to have to specify that a file starts on the first
line. Remove it from the initializer and default it to 1.

Remove the last vestiges where we check for 'line' being unset, that
can't happen, now.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210519183951.3946870-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>