Prebuilt Removal: Investigate remaining indirect prebuilts in Roslyn repo

[dseefeld]

The following prebuilts are being reported as indirect prebuilts in roslyn. These need to be investigated and removed.

• RoslynTools.Lightup.System.Runtime.Loader -  4.3.0
• System.CodeDom -  4.4.0

[NikolaMilosavljevic]

System.CodeDom prebuilt will be automatically removed when the following PR is completed:
#1291

[NikolaMilosavljevic]

RoslynTools.Lightup.System.Runtime.Loader was removed in Roslyn repo on 8/28 with dotnet/roslyn@3e9505e

It was replaced with a reference of System.Runtime.Loader - that one would need to be removed from prebuilts once we sync to new Roslyn tag.

[NikolaMilosavljevic]

Merged the PR (#1317) with a fix for RoslynTools.Lightup.System.Runtime.Loader

[dseefeld]

All Roslyn prebuilts have been removed.