Revert "Update .readthedocs.yaml"

This reverts commit 01c4393.
Sphinx on ReadTheDocs reports an error with converting SVGs to PNGs
during the PDF build.

Signed-off-by: Joe Stringer <joe@cilium.io>

Author: joestringer

Documentation/.readthedocs.yaml

@@ -21,8 +21,3 @@ sphinx:
 python:
   install:
   - requirements: Documentation/requirements.txt
-
-# to allow generating offline documentation
-formats:
-  - pdf
-  - epub