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))* [',']
```

src/black/lines.py

@@ -353,9 +353,9 @@ def has_magic_trailing_comma(
 
         if closing.type == token.RSQB:
             if (
-                closing.parent
+                closing.parent is not None
                 and closing.parent.type == syms.trailer
-                and closing.opening_bracket
+                and closing.opening_bracket is not None
                 and is_one_sequence_between(
                     closing.opening_bracket,
                     closing,
@@ -365,22 +365,7 @@ def has_magic_trailing_comma(
             ):
                 return False
 
-            if not ensure_removable:
-                return True
-
-            comma = self.leaves[-1]
-            if comma.parent is None:
-                return False
-            return (
-                comma.parent.type != syms.subscriptlist
-                or closing.opening_bracket is None
-                or not is_one_sequence_between(
-                    closing.opening_bracket,
-                    closing,
-                    self.leaves,
-                    brackets=(token.LSQB, token.RSQB),
-                )
-            )
+            return True
 
         if self.is_import:
             return True