MXS-2004 Drop support for thread_stack_size

- Cannot be supported with std::thread.
- Unlikely that the default size (8MB) would ever be too small and
  if it is, there is some problem.
- The stack size can be specified using 'ulimit -s' before starting
  MaxScale.

Documentation/Getting-Started/Configuration-Guide.md

@@ -163,13 +163,10 @@ will be used to manage the user connections.
 
 #### `thread_stack_size`
 
-This parameter controls the stack size of the worker threads. The default value
-is 0, which means that the pthread default will be used. The size can be specified
-as explained in detail [here](#sizes).
+Deprecated in 2.3.
 
-```
-thread_stack_size=5Mi
-```
+If you need to explicitly set the stack size, do so using `ulimit -s` before
+starting MaxScale.
 
 #### `auth_connect_timeout`