accumulate: remove Lisp-specific portion (#586)

Elixir-specific portion added:
exercism/exercism#1136

Lisp-specific portion added:
exercism/problem-specifications#98

Elixir-specific portion removed because the `Enum.reduce` was not
necessary:
exercism/problem-specifications#274

It doesn't seem to make much sense to include this Lisp-specific
sentence in non-Lisp tracks.

exercism/problem-specifications#871

Author: petertseng

exercises/accumulate/README.md

@@ -25,9 +25,6 @@ Keep your hands off that collect/map/fmap/whatchamacallit functionality
 provided by your standard library!
 Solve this one yourself using other basic tools instead.
 
-Lisp specific: it's perfectly fine to use `MAPCAR` or the equivalent,
-as this is idiomatic Lisp, not a library function.
-
 
 ## Getting Started