Merge pull request #2821 from bsipocz/CI_RTD_update

CI: remove deprecated/removed RTD config

Author: bsipocz

.readthedocs.yaml

@@ -15,7 +15,6 @@ sphinx:
 # Install regular dependencies.
 # Then, install special pinning for RTD.
 python:
-  system_packages: false
   install:
     - method: pip
       path: .