Align double definition check with spec

Remove weird special cases for private-local fields
and parameter accessor (fields).

One change with the new trait val encoding:

```
scala>  trait T { private[this] var x: String = "1" ; def x(): Int = 1 }
<console>:11: error: method x is defined twice;
  the conflicting variable x was defined at line 11:37
        trait T { private[this] var x: String = "1" ; def x(): Int = 1 }
                                                          ^
```

Whereas:

```
scala>  class T { private[this] var x: String = "1" ; def x(): Int = 1 }
defined class T
```

Before, both the `class` and `trait` definition were accepted.
(Because there is no accessor for a private[this] val/var,
and a MethodType does not match the type of a value.)

(Dotty accepts neither the class or the trait definition.)

0 files changed, 0 insertions, 0 deletions