docs: Remove python.sytem_packages from ReadTheDocs config

* ReadTheDocs is removing the "use system packages" feature on
  2023-08-29. The user should fully specify the docs environment, which
  for pyhf is already done, but pyhf was following the default template
  which included the uses of system_packages.

.readthedocs.yaml

@@ -28,4 +28,3 @@ python:
       path: .
       extra_requirements:
         - docs
-  system_packages: true