Use default interpreter value of system Python

The research-template Dockerfile now fakes/redirects
/usr/bin/python to the virtualenv Python, allowing us to
change this redirection in the future without modifying the
devcontainer config.

.devcontainer/devcontainer.json

@@ -39,8 +39,8 @@
                 "files.autoSaveDelay": 1000,
                 "git.autofetch": true,
                 "python.analysis.extraPaths": [".devcontainer/ehrql-main/"],
-                "python.defaultInterpreterPath": "/opt/venv/bin/python3.10",
                 "python.terminal.activateEnvironment": true,
+                "python.terminal.activateEnvInCurrentTerminal": true,
                 "window.autoDetectColorScheme": true
             }
         }