lang: ast, funcs: Remove the secret channel from call

This removes the secret channel from the call function. Having it made
it more complicated to write new function engines, and it's not clear
why it was even needed in the first place. It seems that even the
current generation of function engines work just fine without it.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/interpret_test/TestAstFunc1/changing-func.txtar

@@ -25,6 +25,8 @@ Edge: FuncValue -> if # a
 Edge: FuncValue -> if # a
 Edge: FuncValue -> if # b
 Edge: FuncValue -> if # b
+Edge: call -> callSubgraphOutput # dummy
+Edge: call -> callSubgraphOutput # dummy
 Edge: const: bool(false) -> if # c
 Edge: const: bool(true) -> if # c
 Edge: if -> call # fn
@@ -35,6 +37,8 @@ Vertex: FuncValue
 Vertex: FuncValue
 Vertex: call
 Vertex: call
+Vertex: callSubgraphOutput
+Vertex: callSubgraphOutput
 Vertex: const: bool(false)
 Vertex: const: bool(true)
 Vertex: if