Remove matrix_synapse_role_executed and various tasks that check it

This was useful when the order of these roles in relation to Synapse
mattered (when we were injecting stuff into Synapse variables during
runtime). This is no longer the case since 0ea7cb5d18, so all of
this can be removed.

roles/custom/matrix-bridge-mautrix-signal/tasks/setup_install.yml

@@ -1,13 +1,5 @@
 ---
 
-# If the matrix-synapse role is not used, `matrix_synapse_role_executed` won't exist.
-# We don't want to fail in such cases.
-- name: Fail if matrix-synapse role already executed
-  ansible.builtin.fail:
-    msg: >-
-      The matrix-bridge-mautrix-signal role needs to execute before the matrix-synapse role.
-  when: "matrix_synapse_role_executed | default(False)"
-
 - name: Ensure Mautrix Signal image is pulled
   community.docker.docker_image:
     name: "{{ matrix_mautrix_signal_docker_image }}"