e2e: change default for --provider

The empty string was the default and then triggered a special
warning. There's no good reason for that behavior, so now the special
handling for "unset provider" is gone and "skeleton" is the non-empty
default for the value.

test/e2e/framework/provider.go

@@ -64,11 +64,8 @@ func init() {
 	RegisterProvider("skeleton", func() (ProviderInterface, error) {
 		return NullProvider{}, nil
 	})
-	// The empty string also works, but triggers a warning.
-	RegisterProvider("", func() (ProviderInterface, error) {
-		Logf("The --provider flag is not set.  Treating as a conformance test.  Some tests may not be run.")
-		return NullProvider{}, nil
-	})
+	// The empty string used to be accepted in the past, but is not
+	// a valid value anymore.
 }
 
 // SetupProviderConfig validates the chosen provider and creates