Fix systemd_path being undefined breakage

Regression since #681 (Github Pull Request).

Fixes #715 (Github Issue).

roles/matrix-dynamic-dns/tasks/uninstall.yml

@@ -2,7 +2,7 @@
 
 - name: Check existence of matrix-dynamic-dns service
   stat:
-    path: "{{ systemd_path }}/matrix-dynamic-dns.service"
+    path: "{{ matrix_systemd_path }}/matrix-dynamic-dns.service"
   register: matrix_dynamic_dns_service_stat
 
 - name: Ensure matrix-dynamic-dns is stopped
@@ -14,7 +14,7 @@
 
 - name: Ensure matrix-dynamic-dns.service doesn't exist
   file:
-    path: "{{ systemd_path }}/matrix-dynamic-dns.service"
+    path: "{{ matrix_systemd_path }}/matrix-dynamic-dns.service"
     state: absent
   when: "matrix_dynamic_dns_service_stat.stat.exists"