Remove the system user set if it is not supported

[unkcpz]

For the new docker stack, aiidalab can not support changing system user because the user can only change upon https://jupyter-docker-stacks.readthedocs.io/en/latest/using/common.html#user-related-configurations and start container as root. However, start AiiDAlab container as root will run all beforenotebook.d scripts as root and it will fail the start of PostgreSQL services as reported in aiidalab/aiidalab-docker-stack#297 (comment)

There are two options:

• Remove the system user setup from profile setup, the profile object still contains this information but it just not shown in the profile edit so user is not allowed to change it.
• Support configure system user.