internal/impl: remove the unreachable line of code

Change-Id: Ie929703894858848c8465e0889d4fab04c1facc9
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/463795
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
diff --git a/internal/impl/convert.go b/internal/impl/convert.go
index 11a6128..185ef2e 100644
--- a/internal/impl/convert.go
+++ b/internal/impl/convert.go
@@ -59,7 +59,6 @@
 	default:
 		return newSingularConverter(t, fd)
 	}
-	panic(fmt.Sprintf("invalid Go type %v for field %v", t, fd.FullName()))
 }
 
 var (