Don't Box<ASTNode> in SQLStatement

This used to be needed when it was a variant in the ASTNode enum itself.

tests/sqlparser_generic.rs

@@ -38,7 +38,7 @@ fn parse_where_delete_statement() {
                     op: Eq,
                     right: Box::new(SQLValue(Value::Long(5))),
                 },
-                *selection.unwrap(),
+                selection.unwrap(),
             );
         }