Darkness vignette effect is shown in Spectator mode

This issue was originally discussed in MC-73730, but has been split into this ticket for clarity.

The bug

The darkness effect seen around the edges of the screen also appears in Spectator mode. This is undesirable, as Spectator mode is meant to be mostly free of visual hindrances (refer to MC-71530, MC-213743 and other tickets with the spectator-visual-impairments label and their triages/resolutions).

How to reproduce

As this effect is rather subtle and hard to see, the following reproduction steps rely on MC-73730.

1. Use Fancy/Fabulous! graphics (so MC-241321 does not interfere) and be in Creative mode

2. Enter a dark area

3. Now switch to Spectator mode

4. Press F1

   1. Notice how the screen ends up appearing slightly brighter as a result

Expected results

The darkness would be completely absent in Spectator mode, since said gamemode is intended to give as clear view of the world as possible.

Actual results

It still shows up in Spectator (hence the change with F1).