Revert "Restore self-building on ARM for Synapse for now"

This reverts commit 7d211b71e2.

The multi-arch images are now available, as discussed in #699 (Github Issue).

group_vars/matrix_servers

@@ -921,7 +921,7 @@ matrix_client_element_jitsi_preferredDomain: "{{ matrix_server_fqn_jitsi if matr
 #
 ######################################################################
 
-matrix_synapse_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"
+matrix_synapse_container_image_self_build: "{{ matrix_architecture not in ['arm32', 'arm64', 'amd64'] }}"
 
 # When ma1sd is enabled, we can use it to validate email addresses and phone numbers.
 # Synapse can validate email addresses by itself as well, but it's probably not what we want by default when we have an identity server.