Delete Xamarin.Common.targets on AppVeyor
Delete Xamarin.Common.targets on AppVeyor

This is a workaround for a Xamarin build script bug specific to
AppVeyor.  For more details see:

http://help.appveyor.com/discussions/problems/4569

aaronmjacobs/openal-soft,aaronmjacobs/openal-soft