[NuGet] Remove forward slash imports when updating a package.

Fixes Bug 20610 - Xamarin Forms samples will not build on Windows
even after package update.
https://bugzilla.xamarin.com/show_bug.cgi?id=20610

On uninstalling or updating a NuGet package, .targets file imports
are removed from the project if they contain forward slashes and if
they contain backslashes. This allows the project to fix itself
without requiring manual editing of the MSBuild project file.