ci: Remove "default" test env

It is unclear what the point is of maintaining a "default", the meaning
of which is unclear.

ci/README.md

@@ -20,12 +20,6 @@ requires `bash`, `docker`, and `python3` to be installed. To install all require
 sudo apt install bash docker.io python3
 ```
 
-To run the default test stage,
-
-```
-./ci/test_run_all.sh
-```
-
 To run the test stage with a specific configuration,
 
 ```