Remove unused build-tools/timing.

[jpobst]

Context: dotnet/android-tools#220
Context: #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.

[jonathanpeppers]

So, I think this thing was created before .binlog files existed, as a custom MSBuild logger for timings. If we ever needed this information again, we'd probably just use MSBuild.StructuredLogger to open .binlog files instead.