readthedocs.yml: Remove discontinued key "python.system_packages"

Error was:

> Problem in your project's configuration. Invalid configuration option
> "python.system_packages". This configuration key has been deprecated
> and removed. Refer to https://blog.readthedocs.com/use-system-packages-deprecated/
> to read more about this change and how to upgrade your config file.

Build log at https://readthedocs.org/projects/ansi2html/builds/21826996/ .

Upstream docs at https://blog.readthedocs.com/drop-support-system-packages/ .
Fix may be incomplete and may need a closer look.

readthedocs.yml

@@ -5,7 +5,6 @@ mkdocs:
 
 python:
   version: 3.8
-  system_packages: false
   install:
     - method: pip
       path: .