nixos/filesystems: Remove default = "auto" from fsType

NixOS has traditionally enabled the `ext` family of file systems by
default. Originally, when switching to systemd initrd, we wanted to
transition to making this explicit so that initrds could be made
without `ext`. The problem is that anyone with `fsType = "auto";` for
an `ext` file system in initrd will fail to boot, which is not really
an acceptable regression as we switch to systemd initrd by default.

By removing `default = "auto"` from `fsType`, we rule out the vast
majority of these regressions as eval errors, since most users of
`fsType = "auto"` for ext file systems are using it because of the
default value.

In hindsight, this is probably what #225352 was really about.

nixos/tests/qemu-vm-external-disk-image.nix

@@ -23,6 +23,7 @@ let
 
       fileSystems = {
         "/".device = rootFsDevice;
+        "/".fsType = "ext4";
       };
 
       system.switch.enable = true;