Remove tableView delegates when dealloc.

We recently saw several `EXEC_BAD_ACCESS` crashes related to `UITableView` delegates, and resolved those issues by manually setting delegates to `nil`.

Surprisingly, those `UITableView` delegates are not `weak` properties but `assign`. Ref:
```
@property (nonatomic, assign)   id <UITableViewDataSource> dataSource;
@property (nonatomic, assign)   id <UITableViewDelegate>   delegate;
```