Remove redundant condition from `has_magic_trailing_comma` (#4023)

The second `if` cannot be true at its execution point, because it is
already covered by the first `if`. The condition
`comma.parent.type == syms.subscriptlist` always holds if
`closing.parent.type == syms.trailer` holds, because `subscriptlist`
only appears inside `trailer` in the grammar:

```
trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
subscriptlist: (subscript|star_expr) (',' (subscript|star_expr))* [',']
```