Data flow: Remove recommendation to use unique in Node::getEnclosingCallable()

Author: hvitved

docs/ql-libraries/dataflow/dataflow.md

@@ -98,10 +98,6 @@ Recommendations:
   also work, but the upside of `use-use` steps is that sources defined in terms
   of variable reads just work out of the box. It also makes certain
   barrier-implementations simpler.
-* A predicate `DataFlowCallable Node::getEnclosingCallable()` is required, and in
-  order to ensure appropriate join-orders, it is important that the QL compiler knows
-  that this predicate is functional. It can therefore be necessary to enclose the body
-  of this predicate in a `unique` aggregate.
 
 The shared library does not use `localFlowStep` nor `localFlow` but users of
 `DataFlow.qll` may expect the existence of `DataFlow::localFlowStep` and