Remove defusedxml support and document why

doc/implementation_notes.md

@@ -61,3 +61,11 @@ Images handling
 When possible, images are handled like PDF: rendered on a surface, then saved
 to the filesystem. This ensures that every metadata is removed.
 
+XML attacks
+-----------
+
+Since our thread model conveniently excludes files crafted to specifically
+bypass MAT2, fileformats containing harmful XML are out of our scope.
+But since MAT2 is using [etree](https://docs.python.org/3/library/xml.html#xml-vulnerabilities)
+to process XML, it's "only" vulnerable to DoS, and not memory corruption:
+odds are that the user will notice that the cleaning didn't succeed.

libmat2/office.py

@@ -7,12 +7,8 @@ import zipfile
 import logging
 from typing import Dict, Set, Pattern
 
-try:  # protect against DoS
-    from defusedxml import ElementTree as ET  # type: ignore
-except ImportError:
 import xml.etree.ElementTree as ET  # type: ignore
 
-
 from . import abstract, parser_factory
 
 # Make pyflakes happy