Revert restrictive validation behavior for DM/WM defaults in the X module

The original idea behind this change (described in ticket #11064) was to
improve the assertions to avoid that users of the X server accidentally
forget to configure a DM or WM.

However this caused several issues with setups that require X, but no DM
or WM. The keymap testcases became instable as well as now disabling DMs
needs to be done explicitly.
(see https://github.com/NixOS/nixpkgs/pull/31268#issuecomment-347080036)

In the end the idea behind the change and #11064 was obviously a
mistake, so reverting it completely for now should be fine.