Remove prebuilt dependency on `jnm2.ReferenceAssemblies.net35`, now handled by `Microsoft.NETFramework.ReferenceAssemblies`

[dagood]

• For: Resolve prebuilts for the msbuild repo for 6.0 source-build#2417

The jnm2.ReferenceAssemblies.net35 1.0.1 package is a prebuilt dependency for source-build 6.0:

msbuild/src/StringTools/StringTools.csproj

Line 32 in bbb9655

<PackageReference Include="jnm2.ReferenceAssemblies.net35" PrivateAssets="All" />

msbuild/eng/Packages.props

Line 8 in bbb9655

<PackageReference Update="jnm2.ReferenceAssemblies.net35" Version="1.0.1" />

The package is deprecated: https://www.nuget.org/packages/jnm2.ReferenceAssemblies.net35/1.0.1

I think it can be removed without adding anything else back in, because the SDK will automatically use the official reference package to build the net35 target framework.

Maybe this also needs to be removed to prevent downgrade (or maybe it needs an update to the new version that supports net35):

msbuild/eng/Packages.props

Lines 46 to 48 in bbb9655

	<ItemGroup Condition="'$(DotNetBuildFromSource)' != 'true'">
	<GlobalPackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All"/>
	</ItemGroup>

---

This usage doesn't show up in the prebuilt report--I suppose due to MSBuild.SourceBuild.slnf:

msbuild/src/MSBuildTaskHost/MSBuildTaskHost.csproj

Line 211 in bbb9655

<PackageReference Include="jnm2.ReferenceAssemblies.net35" PrivateAssets="All" />

---

I'm trying out a targeted removal of the reference in src/StringTools/StringTools.csproj for source-build 6.0, but maybe something broader could be done here.

[KirillOsenkov]

The repo currently doesn't build on a machine without .NET 3.5 SP1 reference assemblies installed.

[dagood]

Feel free to revert my PR or add DotNetBuildFromSource conditions in the right places to unblock developer scenarios. I don't have the bandwidth to fix it (including figuring out how to reproduce the error to try out fixes).

[dagood]

I read these comments in a bad order. 😄 I can try a quick PR based on #6966 (comment).