wpctl: fix types in variadic arguments

`wp_object_manager_add_interest()` passes the format string
and the arguments after that to `g_variant_new()`, which
requires a 32-bit integer for "u". Passing a 64-bit integer
will cause problems on certain ABIs.

Furthermore, remove the metadata related interest declaration
from `set_default_prepare()` since the "set-default" command
does not access metadata directly, it uses the "default-nodes-api"
plugin.

Fixes: 7784cfad ("wpctl: support @DEFAULT_{AUDIO,VIDEO}_{SINK,SOURCE}@ as ID ")
Fixes #773