Context: https://github.com/xamarin/xamarin-android-tools/pull/220
Context: https://github.com/xamarin/xamarin-android/pull/8545

We added the ability for consumers of MSBuild NuGet packages to not add the `Microsoft.Build` package to their project.  One project that currently still requires `Microsoft.Build` is `Xamarin.Android.BootstrapTasks`.  

However the only thing that uses `Microsoft.Build` is `TimingLogger` which produced a build performance report for our old Jenkins build system.  As we no longer use Jenkins or this code, removing it allows us to remove the `Microsoft.Build` dependency.

Removes warnings:

```
C:\a\_work\1\s\build-tools\Xamarin.Android.Tools.BootstrapTasks\Xamarin.Android.Tools.BootstrapTasks.csproj : warning NU1701: Package 'Microsoft.Build 17.8.3' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8, .NETFramework,Version=v4.8.1' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project. [C:\a\_work\1\s\Xamarin.Android.BootstrapTasks.sln]
C:\a\_work\1\s\build-tools\Xamarin.Android.Tools.BootstrapTasks\Xamarin.Android.Tools.BootstrapTasks.csproj : warning NU1701: Package 'Microsoft.IO.Redist 6.0.0' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8, .NETFramework,Version=v4.8.1' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project. [C:\a\_work\1\s\Xamarin.Android.BootstrapTasks.sln]
```

This reduces the number of CI reported warnings from 166 to 56.