Remove mention of obsolete --use-keywords in version 2.0 migration doc (

googleapis#635)

Fixes googleapis#630

Co-authored-by: meredithslota <meredithslota@google.com>
Co-authored-by: Anna Cocuzzo <63511057+acocuzzo@users.noreply.github.com>
Co-authored-by: Tianzi Cai <tianzi@google.com>

UPGRADING.md

@@ -32,9 +32,6 @@ python3 -m pip install google-cloud-pubsub[libcst]
 
 * The script `fixup_pubsub_v1_keywords.py` is shipped with the library. It expects
 an input directory (with the code to convert) and an empty destination directory.
-Optionally, the `--use-keywords` switch can be added to generate flattened keyword
-parameters instead of a request dictionary (see the following section for an
-explanation).
 
 ```sh
 $ scripts/fixup_pubsub_v1_keywords.py --input-directory .samples/ --output-directory samples/