qapi: Inline gen_visit_members() into lone caller

qapi: Inline gen_visit_members() into lone caller

Commit 82ca8e46 noticed that we had multiple implementations of
visiting every member of a struct, and consolidated it into
gen_visit_fields() (now gen_visit_members()) with enough
parameters to cater to slight differences between the clients.
But recent exposure of implicit types has meant that we are now
down to a single use of that method, so we can clean up the
unused conditionals and just inline it into the remaining
caller: gen_visit_object_members().

Likewise, gen_err_check() no longer needs optional parameters,
as the lone use of non-defaults was via gen_visit_members().

No change to generated code.

Suggested-by: Markus Armbruster <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1458254921[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>