raspberry-pi: remove DigiAMP+ module

[JamieMagee]

Description of changes

Removes hardware.raspberry-pi."4".digi-amp-plus in favour of the stock iqaudio-dacplus firmware overlay through hardware.raspberry-pi.configtxt.deviceTreeOverlays.

The migration keeps the old autoMuteAmp and unmuteAmp defaults and behaviour. It uses iqaudio-dacplus instead of rpi-digiampplus (because the latter makes unmute_amp disable auto-mute).

Part of #1946.

cc: @doronbehar

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[doronbehar]

In general I am not sure we need to add an entry to this README per every module we convert from a dts source to configtxt module...

[JamieMagee]

In general I am not sure we need to add an entry to this README per every module we convert from a dts source to configtxt module...

I removed the dedicated DigiAMP+ README section. The generic overlay documentation remains in the README, and the migration details stay in the removed-option message.

[doronbehar]

Be specific & explicit! :)

I was also thinking, maybe we can go ahead and perform all of the config.txt removals in a single PR? I don't expect to encounter merge conflicts if we don't edit the RaspberryPi README. I also think we should reconsider whether we want to not document non-nixos specific details in this README, such as in #### DWC2 USB controller and #### PoE HATs.

[JamieMagee]

@doronbehar sounds good! I'll send a PR after this one, to migrate all the remaining removals at once. I can remove the specific hardware notes from the README, and update any mkRemovedOptionModule notes if need be.