Commented out 'invalidation_service' section

I commented out the entire 'invalidation_service' section from
app_config.yml. It _should_ be sufficient to set 'enabled' to false in
that block, in order to prevent the Redis/Resque based invalidation
service from being used inside the postgres trigger for invalidating
cache items, but it's actually easier to just comment out the whole
block. See this portion of the Carto code for reference:

05a05fd695/app/models/user/db_service.rb (L1062-L1070)

The branch we want to go down in that code is the middle one--we want to
end up with `create_function_invalidate_varnish_http` running. That will
create a postgres trigger based on hitting the Varnish server's HTTP
listener, which is running on 6081. (You could have it hit the telnet
port by taking the third branch of that code, but given that telnet
isn't included in later Varnish versions, best not to.)

You want to avoid the first branch of that code, `create_function_invalidate_varnish_invalidation_service`,
because it includes this line:

05a05fd695/app/models/user/db_service.rb (L1601)

That's calling a custom Redis command, `TCH`, which is defined in a repo
that Carto has not open sourced--meaning the 'invalidation service' (as
a Redis job queue for the Resque job runner) can't be used in open
source Carto (unless you reverse engineer the Redis commands it uses.)

config/app_config.yml

@@ -163,14 +163,14 @@ defaults: &defaults
     timeout: 5
     # 'warning' or 'error'
     trigger_verbose: true
-  invalidation_service:
+#  invalidation_service:
-    enabled: false
+#    enabled: false
-    host: '127.0.0.1'
+#    host: '127.0.0.1'
-    port: 3142
+#    port: 3142
-    retries: 5 # number of retries before considering failure
+#    retries: 5 # number of retries before considering failure
-    critical: false # either the failure is considered an error or a warning
+#    critical: false # either the failure is considered an error or a warning
-    timeout: 5 # socket timeout
+#    timeout: 5 # socket timeout
-    trigger_verbose: true
+#    trigger_verbose: true
   redis:
     host: '127.0.0.1'
     port: 6379