Remove Benchmark from Appveyor build.

This takes a long time to run and doesn't trigger any build failures
on a perf regression, so newly-introduced problems are unlikely to be
detected.

Author: bgrainger

.ci/test.ps1

@@ -51,11 +51,3 @@ dotnet test tests\SideBySide\SideBySide.csproj -c Baseline
 if ($LASTEXITCODE -ne 0){
     exit $LASTEXITCODE;
 }
-
-echo "Building Benchmark"
-
-dotnet restore tests\Benchmark\Benchmark.csproj
-dotnet run -p tests\Benchmark\Benchmark.csproj -c Release -f net462
-if ($LASTEXITCODE -ne 0){
-    exit $LASTEXITCODE;
-}