GWin32AppInfo: Remove assertion on the opened registry key

Relax assertion about opened registry key as it may have been removed
in the meantime between enumeration and when opening, or (more likely)
we may not have the required permissions to open the some enumerated
keys (i.e. RegOpenKeyExW fails and returns ERROR_ACCESS_DENIED).

Fixes https://gitlab.com/inkscape/inbox/-/issues/5669
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