flambda-backend: Allow make debug with dune-based build (#1480)

* Allow `make debug` with dune-based build

* Remove old debug_printers rule, which conflicts

* Document how to use the debugger

Author: goldfirere

tools/dune

@@ -91,16 +91,4 @@
   (libraries ocamlcommon)
   (modules debug_printers))
 
-(rule
-  (target debug_printers)
-  (deps debug_printers.ml %{cmo:debug_printers})
-  (action
-    (with-stdout-to %{target}
-      (progn
-        ; Resorting to Bash instead of the built-in [echo] action because I
-        ; couldn't find a better way to get an absolute path out of Dune
-        (bash "echo load_printer \\\"$(realpath %{cmo:debug_printers})\\\"")
-        (with-stdin-from debug_printers.ml
-          (run awk "{ print \"install_printer Debug_printers.\" $2 }"))))))
-
 ; ocamlcp, ocamloptp and ocamlprof are not currently supported.