Remove outdated reference in docs to pillow-heif (#181)

Author: Stormheg

docs/reference.rst

@@ -334,8 +334,6 @@ Here's a full list of operations provided by Willow out of the box:
 
 .. method:: save_as_avif(file, quality=80, lossless=False)
 
-    (requires the `pillow-heif <https://pypi.org/project/pillow-heif/>`_ library)
-
     Saves the image to the specified file-like object in AVIF format.
     When saving with `lossless=True`, no `chroma subsampling <https://en.wikipedia.org/wiki/Chroma_subsampling>` is used (4:4:4 instead of the default 4:2:0).