Remove unnecessary variable

The /data is hardcoded in the container

roles/matrix-bot-maubot/templates/systemd/matrix-bot-maubot.service.j2

@@ -21,7 +21,7 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-bot-maubot \
             -e UID={{ matrix_user_uid }} \
             -e GID={{ matrix_user_gid }} \
             --read-only \
-            -v {{ matrix_bot_maubot_data_path }}:{{ matrix_bot_maubot_container_data_dir }}:z \
+            -v {{ matrix_bot_maubot_data_path }}:/data:z \
 			{% for arg in matrix_bot_maubot_container_extra_arguments %}
 			    {{ arg }} \
 			{% endfor %}