prometheus-xmpp-alerts: Fix build by removing unittestCheckHook argument

In #185430 the `unittestCheckHook` argument was introduced, causing the
derivation to fail building since the `unittestCheckHook` is not passed.
However the original PR already used the `unittestCheckHook` from
`python3Packages`.

-1
-1
pkgs/servers/monitoring/prometheus/xmpp-alerts.nix
··· 2 , fetchFromGitHub 3 , python3Packages 4 , prometheus-alertmanager 5 - , unittestCheckHook 6 }: 7 8 python3Packages.buildPythonApplication rec {
··· 2 , fetchFromGitHub 3 , python3Packages 4 , prometheus-alertmanager 5 }: 6 7 python3Packages.buildPythonApplication rec {