Remove redundant immutable package pin

Drop the explicit System.Collections.Immutable PackageReference from MicroBenchmarks since the project still restores and builds for the supported modern TFMs without it, and the remaining net472 issue is unrelated.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: LoopedBard3

src/benchmarks/micro/MicroBenchmarks.csproj

@@ -100,7 +100,6 @@
     <PackageReference Include="System.IO.Hashing" Version="$(SystemVersion)" />
     <PackageReference Include="System.Net.Http.WinHttpHandler" Version="$(SystemVersion)" Condition="'$(OS)' == 'Windows_NT'" />
     <PackageReference Include="Utf8Json" Version="1.3.7" />
-    <PackageReference Include="System.Collections.Immutable" Version="9.0.*-*" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' != '' and '$(TargetFrameworkIdentifier)' == '.NETFramework'">
     <PackageReference Include="System.Memory" Version="4.6.3" />