Visual Studio opens my class-file (.cs) in the designer mode

I've created a class that extends DbConnection in a brand new project.

public class FakeDbConnection : DbConnection { ... }

In the Solution Explorer the class looks like this: enter image description here

And when double-clicking it wants to open it in design mode which won't work. Opening up the .csproj-file reveals the problem

<ItemGroup>
  <Compile Include="FakeADO\FakeDbConnection.cs">
    <SubType>Component</SubType>
  </Compile>
</ItemGroup>

Even if I remove the SubType tag VS2010 immediately re-adds it. Very annoying.

How can I stop VS2010 from opening up my .cs file in designer mode and just open it up as a regular code file?