qapi: Remove visit_start_alternate() parameter promote_int

qapi: Remove visit_start_alternate() parameter promote_int

Before the previous commit, parameter promote_int = true made
visit_start_alternate() with an input visitor avoid QTYPE_QINT
variants and create QTYPE_QFLOAT variants instead.  This was used
where QTYPE_QINT variants were invalid.

The previous commit fused QTYPE_QINT with QTYPE_QFLOAT, rendering
promote_int useless and unused.

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Message-Id: <20170607163635[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>