nixos/tests/herbstluftwm: fix timeout that was given in seconds

Hydra accepts timeouts as value of seconds after which the test is
terminated / considered failed. Using the value 30 here has the effect
that the test was terminate after 30 seconds. That time might be
sufficient for the test execution itself but it has another downside:

Jobs on hydra inherit the timeout of their parent. In this case all the
builds that are a dependency of the herbstluftwm test *must* finish
(each) within 30s. And since not all of the dependencies are cached in
the binary cache this could lead to an issue with pacakges that take
longer than 30s to build at the time when the herbstluftwm test is built
by hydra.

It is best to not set the timeout here and let hydra deal with it.  Our
default timeout for builds is two hours which is more than sufficient
for most builds and tests. If the test fails we will spent ~2h doing
something or nothing at worst but at least we wont kill the build just
because a dependency wasn't fullfilled already.

nixos/tests/herbstluftwm.nix

@@ -3,7 +3,6 @@ import ./make-test-python.nix ({ lib, ...} : {
 
   meta = {
     maintainers = with lib.maintainers; [ thibautmarty ];
-    timeout = 30;
   };
 
   machine = { pkgs, lib, ... }: {