nixos/test: remove the stateVersion statement from the test-instrumentation

We set stateVersion to `mkDefault 18.03` in
`nixos/modules/testing/test-instrumentation.nix` and in
`modules/installer/cd-dvd/installation-cd-base.nix`.

Accessing the stateVersion in the module system from within the tests
results in the following error:
> The unique option `system.stateVersion' is defined multiple times, in
> `nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-base.nix' and
> `nixpkgs/nixos/modules/testing/test-instrumentation.nix'.

There are other tests that use it as well. Namely the radicale test also
verifies behaviour between state versions is as expected. It switches a
package default value. Others switched on the state directory default.
It seems like having the timesyncd switch as part of every rendered
activationScript might cause this weird error.

Removing this line seems like a reasonable thing to do since we actually
set the default to the very same value in the module system. This line
should have been no-op besides the issue that we've two statements
setting it in this very specific case.