gdm/util: Do not run dispose on user verifier

gdm/util: Do not run dispose on user verifier

This is not a good habit anywhere and especially in JS code, I assume
the rationale for this was to ensure that cached instances of the proxy
were cleared, but this did not work properly (as it happened on
finalize), and it would still require [1].

So let's just avoid this and assume that the getters just return what we
expect.

[1] https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/321

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3885>