chore: Stop creating multiprocessing.rst (#241)

It's included in README.rst now (to make navigation saner).

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery-sqlalchemy/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes #240 🦕

Author: Jim Fulton

owlbot.py

@@ -34,7 +34,7 @@
 )
 s.move(templated_files, excludes=[
     # sqlalchemy-bigquery was originally licensed MIT
-    "LICENSE"
+    "LICENSE", "docs/multiprocessing.rst"
 ])
 
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