[mono-sdks] <DownloadUri/> should not run during design-time builds (d…

…otnet#2439)

Fixes: dotnet#2350

Most of the logic in `mono-runtimes.csproj` was happening during the
`CoreCompile` target.  The problem with this is that design-time
builds in Visual Studio runs `CoreCompile`.  The `Task.Wait()` in the
`<DownloadUri/>` task causes a hang within Visual Studio.

It turns out we could just completely remove the following target:

	<Target Name="CoreCompile"
	    DependsOnTargets="_Build">
	</Target>

`_Build` still runs appropriately. `_BuildUnlessCached` will call
`ForceBuild` or `_Build` depending if things are cached.

src/mono-runtimes/mono-runtimes.targets

@@ -627,7 +627,4 @@
     <Delete Files="@(_StampFiles)" />
   </Target>
 
-  <Target Name="CoreCompile"
-      DependsOnTargets="_Build">
-  </Target>
 </Project>