Fixes: https://github.com/xamarin/xamarin-android/issues/3132

Context: https://github.com/xamarin/monodroid/commit/b9349491a141b5265e3d48eec9af589c8c5cc793

`Xamarin.Android.Cecil.dll` was created to avoid potential conflicts
and versioning issues with the copy of Cecil that Xamarin Studio was
using at the time; from April 2013:

> The issue is that when Xamarin Studio builds MSBuild-based
> projects, it does so through an intermediary
> `MonoDevelop.Projects.Formats.MSBuild.exe` process, which is
> responsible for using the `Microsoft.Build.*` assemblies to
> actually build the project.
> 
> The problem is that the directory containing
> `MonoDevelop.Projects.Formats.MSBuild.exe` --
> `/Applications/Xamarin Studio.app/Contents/MacOS/lib/monodevelop/bin`
> -- contains its own copy of `Mono.Cecil.dll`, and _that_ is the
> version that was used!
>
> Meaning that in order to fix a Xamarin.Android bug, we would need
> an updated Xamarin Studio release.

Considering our migration to .NET, and how infrequently we change our
`Mono.Cecil` reference these days, these potential issues are likely
no longer a concern.

Our unique `Xamarin.Android.Cecil.dll` copy has been removed in favor
of `Mono.Cecil` package references in all projects that depend on it.