Remove project dependencies when removing a project from a solution.

This commit fixes #6198.

When a project is removed from a solution using the `sln remove`
command, any projects in the solution with a project dependency (note:
this is different from a project reference) on the project should have
the project removed as a dependency.

The fix is to scan the projects in the solution and remove any
dependencies on the projects being removed.  If the dependencies section
is empty after the remove, we skip serialization of the section like
Visual Studio does.

TestAssets/TestProjects/TestAppWithSlnProjectDependencyToRemove/App/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace App
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}