Clean unused DisplayApp messages

[mark9064]

There are some old left over messages from before RestoreBrightness became NotifyDeviceActivity, and also some other ones that are well before my time. Since screens pull the time directly from the time controller, removing UpdateDateTime seems justified. And I can't think of anything that'd want to send DimScreen to DisplayApp, so I removed this too (the message is currently unused)

[github-actions[bot]]

Build size and comparison to main:

Section	Size	Difference
text	378772B	-128B
data	948B	0B
bss	63488B	0B

[mark9064]

Simplified RestoreBrightness() away too