Remove WEB_CONCURRENCY "auto" from tuning_performance_for_deployment (rails#52545)

Puma 5+ parses environment variable `WEB_CONCURRENCY` directly so
it can't be extended with "auto"

Follow up to:
* rails#52541
* rails#52533

Author: denys-chaikovskyi

guides/source/tuning_performance_for_deployment.md

@@ -122,16 +122,7 @@ or if the server is running multiple applications, to how many cores you want th
 If you only use one thread per worker, then you can increase it to above one per process to account for when workers are
 idle waiting for I/O operations.
 
-In the default generated configuration, it is set to use 1 worker.
-You can also modify it by setting the `WEB_CONCURRENCY` environment variable.
-
-* Setting `WEB_CONCURRENCY` to a specific number will configure Puma to use that many workers.
-* Setting `WEB_CONCURRENCY` to "auto" will make Puma use all available processor cores on the server, as determined by
-  the `Concurrent.available_processor_count` helper.
-
-Please note that using "auto" might result in incorrect configurations on some cloud hosts with shared CPUs or platforms
-that inaccurately report CPU counts.
-
+You can configure number of Puma workers by setting the `WEB_CONCURRENCY` environment variable.
 
 ### YJIT