timeLimitsManager: Remove a redundant fallback value

timeLimitsManager: Remove a redundant fallback value

This code was originally copied from `lightbox.js`, where the fallback
is potentially useful because the duration is provided as an argument.
The `timeLimitsManager` uses a constant as the duration, though, so the
fallback is just confusing.

Spotted in https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3655#note_2369352

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3655>