Remove parts of PreferencesModel that are confusing because of Tandem.OPT_OUT.

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This is an example of why it's potentially dangerous to simply add migration rules without understanding/documenting why they are being added.

While working on https://github.com/phetsims/phet-io/issues/1963#event-10350028993, we wondered about these rules that appear in many sims:

    new TandemFragmentDelete( `${simName}.general.model.preferencesModel.visualModel.colorProfileProperty` ),
    new TandemFragmentDelete( `${simName}.general.model.preferencesModel.localizationModel.localeProperty` ),

And there are similar sim-specific rules, like these for beers-law-lab:

    new TandemFragmentDelete( `${simName}.general.model.preferencesModel.simulationModel.showSoluteAmountProperty` ),
    new TandemFragmentDelete( `${simName}.general.model.preferencesModel.simulationModel.showSolutionVolumeProperty` ),

Sims have been using the modelLinkables option to specify Properties that should appear under ${simName}.general.model.preferencesModel.simulationModel. For example, in BLLSim.ts:

      preferencesModel: new PreferencesModel( {
        simulationOptions: {
...
            modelLinkables: [
              { property: BLLPreferences.showSoluteAmountProperty },
              { property: BLLPreferences.showSolutionVolumeProperty }
            ]
          } ]
        }
      } ),

That feature was apparently short-circuited by work in https://github.com/phetsims/phet-io/issues/1913. Those link are no longer being created because PreferencesModel was changed to opt-out of ALL instrumentation. In PreferencesModel.ts:

        tandem: Tandem.OPT_OUT, // Uninstrumented for now, but keep the file's instrumentation just in case, see https://github.com/phetsims/phet-io/issues/1913

We also noticed that occurrences of modelLinkables are now incomplete -- for example, beers-law-lab is now missing 2 preferences Properties (beakerUnitsProperty and concentrationMeterUnitsProperty). And newer sims have been ignoring modelLinkables altogether (it's only used in 5 sims).

We concluded that we should: (1) Remove usages of modelLinkables (2) remove modelLinkables from the API, because keeping dead/unexercised code around is confusing, and (in this case) not likely to provide much benefit. (3) Clearly/consistently document the rules related to this in all migration.ts files.

And one additional task that I discovered: (4) Since PreferencesModel is opting out of ALL instrumentation, are there other parts of the API that are like modelLinkables, and should also be removed?

I'll handle (1) above. Assigning to @zepumph and @samreid to handle (2), (3), and (4).

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In the above commits, I removed usages of modelLinkables.

I also added this temporary assertion to PreferencesModel addPhetioLinkedElementsForModel, to prevent further uses of modelLinkables before this GitHub issue is completed:

      //TODO https://github.com/phetsims/joist/issues/938 remove the modelLinkables options
      assert && assert( !customPreference.modelLinkables, 'modelLinkables is not supported, see https://github.com/phetsims/joist/issues/938' );

[samreid]

@zepumph there are 6 occurrences of modelLinkables in 1 file. Should I remove them? Will any migration rules be necessary here? I thought not, but wasn't sure why it was mentioned above.

[zepumph]

Sounds good. I'm still not too interested in pulling out all the addLinkedElement logic from PreferencesModel, but please do tell me if you want me to do it and I will gladly.

[samreid]

@jbphet @zepumph and I discussed this today and feel it is in good shape to be closed.