flambda-backend: Remove non_null_value layout (#2659)

* Revert "`Non_null_value` layout (#2480)"

This reverts commit cb697c0.

* Fixes

---------

Co-authored-by: Diana Kalinichenko <dkalinichenko@janestreet.com>

Author: dkalinichenko-js

testsuite/tests/typing-layouts-non-null-value/basics.ml

@@ -32,16 +32,6 @@ Error: Layout void is more experimental than allowed by the enabled layouts exte
        You must enable -extension layouts_alpha to use this feature.
 |}]
 
-type t_non_null_value : non_null_value;;
-
-[%%expect{|
-Line 1, characters 24-38:
-1 | type t_non_null_value : non_null_value;;
-                            ^^^^^^^^^^^^^^
-Error: Layout non_null_value is more experimental than allowed by the enabled layouts extension.
-       You must enable -extension layouts_alpha to use this feature.
-|}]
-
 (***************************************)
 (* Test 1: annotation on type variable *)
 

testsuite/tests/typing-layouts/annots.ml

@@ -31,15 +31,6 @@ Error: Layout void is more experimental than allowed by the enabled layouts exte
        You must enable -extension layouts_alpha to use this feature.
 |}];;
 
-type t_non_null_value : non_null_value;;
-[%%expect{|
-Line 1, characters 24-38:
-1 | type t_non_null_value : non_null_value;;
-                            ^^^^^^^^^^^^^^
-Error: Layout non_null_value is more experimental than allowed by the enabled layouts extension.
-       You must enable -extension layouts_alpha to use this feature.
-|}]
-
 (******************************************************************)
 (* Test 1: Allow non-representable function args/returns in types *)