[AST] Delete the unused FileUnitKind::Derived.

The last use of this went away a loooooong time ago. NFC.

lib/AST/Module.cpp

@@ -835,8 +835,7 @@ lookupOperatorDeclForName(const FileUnit &File, SourceLoc Loc, Identifier Name,
 {
   switch (File.getKind()) {
   case FileUnitKind::Builtin:
-  case FileUnitKind::Derived:
-    // The Builtin module declares no operators, nor do derived units.
+    // The Builtin module declares no operators.
     return nullptr;
   case FileUnitKind::Source:
     break;