html cleaner assert parent is not None AssertionError

The html cleaner raise AssertionError when both root tag and child tag in kill_tags set.

Reproduce:
```
from lxml.html.clean import Cleaner

html_cleaner = Cleaner(kill_tags=['pre', 'code'])

content = '<pre><code>xxx</code></pre>'
print(html_cleaner.clean_html(content))
```

Output:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "src/lxml/html/clean.py", line 520, in lxml.html.clean.Cleaner.clean_html
  File "src/lxml/html/clean.py", line 394, in lxml.html.clean.Cleaner.__call__
  File "/Users/kk/.pyenv/versions/rssant/lib/python3.7/site-packages/lxml/html/__init__.py", line 339, in drop_tree
    assert parent is not None
AssertionError
```

Expected output:
```
<div></div>
```

I tested in Python 3.7.4, lxml-4.3.3 and lxml-4.4.1.

Python : sys.version_info(major=3, minor=7, micro=4, releaselevel='final', serial=0)
lxml.etree : (4, 4, 1, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)

The bug seems similar to https://bugs.launchpad.net/lxml/+bug/773715, but occurs in different line.