Remove outdated sub-workflow documentation.

src/user-guide/running-workflows/dynamic-behaviour.rst

@@ -14,34 +14,4 @@ Broadcast settings targeting a specific cycle point expire as the workflow
 moves on. Otherwise they persist for lifetime of the run, and across restarts,
 unless cleared with another invocation of the command.
 
-
-.. _Sub-Workflows:
-
-Sub-Workflows
--------------
-
-A single Cylc workflow can have multiple cycling sequences in the graph,
-but cycles can't be nested. If you need *cycles within cycles* - e.g. to
-iterate over many files generated by each run of a cycling task - options are:
-
-- Parameterize the sub-cycles
-
-  - This is easy but it makes more tasks-per-cycle, which is the primary
-    determinant of workflow size and scheduler efficiency (this has a much
-    smaller impact on Cylc 8, however)
-
-- Use a separate sub-workflow inside a main-workflow task, to run the sub-cycle
-  tasks at each main-workflow cycle point
-
-  - This is very efficient, but monitoring and run-directory housekeeping are
-    be more complicated because it creates multiple workflows and run directories
-
-Sub-workflows must be started with ``--no-detach`` so that the containing task
-does not finish until the sub-workflow does, and they should be non-cycling
-or have a ``final cycle point`` so they don't keep on running indefinitely.
-
-Sub-workflow names should normally incorporate the main-workflow cycle point
-so that successive sub-workflows can run concurrently if necessary and do not
-compete for the same workflow run directory. This generates a new sub-workflow
-run directory for every main-workflow cycle point, so you may need housekeeping
-tasks to extract outputs from each sub-workflow run and tidy up its directories.
+.. TODO: document sub-workflows