qapi: remove qapi_specific_outputs from meson.build

qapi: remove qapi_specific_outputs from meson.build

There is no more QAPI files that need to be compiled per target, so we
can remove this. qapi_specific_outputs is now empty, so we can remove
the associated logic in meson.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-ID: <20250522190542.588267-12-pierrick.bouvier@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>