Windows Forms VB project template should have implicit imports instead of explict

* .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use `dotnet --info`)
* Have you experienced this same bug with .NET Framework?: Yes/No

**Problem description:**

Unlike the VB Console App and WPF App, the Windows Forms template has some (but not all) "explicit" imports. These should be moved to the Windows Desktop SDK similar to the rest of the imports to keep the project file small by default.

``` XML

WinExe
netcoreapp3.0
WindowsFormsApp3
WindowsFormsApp3.Form1
true

```