Removed duplicate Destination class

I was experimenting with `PdfFileMerger` and when I tried to write the data to a file the following exception was raised:

```pytb
Traceback (most recent call last):
  File "pypdf2/PyPDF2/merger.py", line 194, in write
    self._write_bookmarks()
  File "pypdf2/PyPDF2/merger.py", line 280, in _write_bookmarks
    self._write_bookmarks(b, last_added)
  File "pypdf2/PyPDF2/merger.py", line 292, in _write_bookmarks
    last_added = self.output.addBookmarkDestination(b, parent)
  File "pypdf2/PyPDF2/pdf.py", line 442, in addBookmarkDestination
    parent.addChild(destRef, self)
AttributeError: 'Destination' object has no attribute 'addChild'
```
After debugging this for a while I noticed that there are two different `Destination` classes in PyPDF2 defined in the following modules: `PyPDF2.generic` and `PyPDF2.pdf`. The difference between them is that the one defined in `generic` module inherits from `TreeObject` and has a few extra methods (`getDestArray` and `writeToStream`)  whereas the one defined in `pdf` modules inherits from `DictionaryObject`.

The `addBookmarkDestination` method expects that `parent` has `addChild` method, which in turn is defined in `TreeObject` class. Thus, in order to fix this issue, I removed `Destination` class defined in `PyPDF2.pdf` module.