[🔥AUDIT🔥] Mark interfaces as enabled! (#67)

🖍 _This is an audit!_ 🖍

## Summary:
We've completed support for interfaces!  Fragments are still pending,
but interfaces by themselves should be usable.  But I forgot to actually
remove the flag they were behind.  In this commit I do!

Issue: https://khanacademy.slack.com/archives/C01120CNCS0/p1630012808004100

## Test plan:
make check


Author: benjaminjkraft

Auditors: aberkan, csilvers, dnerdy, MiguelCastillo

Required Reviewers: 

Approved by: 

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/67

generate/convert.go

@@ -256,10 +256,6 @@ func (g *generator) convertDefinition(
 		return goType, nil
 
 	case ast.Interface, ast.Union:
-		if !g.Config.AllowBrokenFeatures {
-			return nil, errorf(pos, "not implemented: %v", def.Kind)
-		}
-
 		implementationTypes := g.schema.GetPossibleTypes(def)
 		goType := &goInterfaceType{
 			GoName:          name,