Remove `stateVersion`, `username`, and `homeDirectory` as they can be
set in the configuration directly. Together with the previous commit,
this makes setting `stateVersion` explicitly mandatory.
Also replace `configuration` by `modules`, which expects a list of
Home Manager modules.
Also remove `system` which was made useless by #3028.
The user should always explicitly set the state version they wish to
use. Indeed, the configuration generated by the Home Manager install
script has set this option for a long time. This removal should
therefore not affect many users.
If the keyboard configuration is an empty set, don't run the setxkbmap
service.
The default values for all keyboard options are null or empty so long
as the state version is set to 19.09 or higher (21.05 being the latest
version).
