Remove FIX ME comment about Synapse's `cache_autotuning`

`docs/maintenance-synapse.md` and `roles/custom/matrix-synapse/defaults/main.yml`
already contains documentation about these variables and the default values we set.

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -812,10 +812,6 @@ caches:
   #
   sync_response_cache_duration: {{ matrix_synapse_caches_sync_response_cache_duration | to_json }}
 
-  #FIX ME add documentation notes about these settings or remove this note.
-  # Check upstream https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#caching
-  # for information for now
-  #
   cache_autotuning:
     max_cache_memory_usage: {{ matrix_synapse_cache_autotuning_max_cache_memory_usage | to_json }}
     target_cache_memory_usage: {{ matrix_synapse_cache_autotuning_target_cache_memory_usage | to_json }}