Remove gen_dot_merlin flag

It's no longer necessary after switching to the new Exe api. The new API will
not generate a .merlin file for utop.

src/gen_rules.ml

@@ -780,7 +780,7 @@ module Gen(P : Params) = struct
         ~preprocessor_deps
         ~lint:exes.buildable.lint
         ~js_of_ocaml:exes.buildable.js_of_ocaml
-        ~has_dot_merlin:exes.buildable.gen_dot_merlin
+        ~has_dot_merlin:true
     in
 
     { Merlin.

src/jbuild.ml

@@ -464,7 +464,6 @@ module Buildable = struct
     ; ocamlc_flags             : Ordered_set_lang.Unexpanded.t
     ; ocamlopt_flags           : Ordered_set_lang.Unexpanded.t
     ; js_of_ocaml              : Js_of_ocaml.t
-    ; gen_dot_merlin           : bool
     }
 
   let modules_field name =
@@ -502,7 +501,6 @@ module Buildable = struct
       ; ocamlc_flags
       ; ocamlopt_flags
       ; js_of_ocaml
-      ; gen_dot_merlin = true
       }
 
   let single_preprocess t =

src/jbuild.mli

@@ -142,9 +142,6 @@ module Buildable : sig
     ; ocamlc_flags             : Ordered_set_lang.Unexpanded.t
     ; ocamlopt_flags           : Ordered_set_lang.Unexpanded.t
     ; js_of_ocaml              : Js_of_ocaml.t
-    ; (** [true] except for on-demand utops, to avoid generation
-          [.utop/.merlin] files everywhere. *)
-      gen_dot_merlin           : bool
     }
 
   (** Preprocessing specification used by all modules or [No_preprocessing] *)

src/super_context.ml

@@ -290,7 +290,7 @@ module Libs = struct
         ~requires:(Lib.Compile.requires lib)
         ~libraries
         ~dep_kind
-        ~has_dot_merlin:conf.buildable.gen_dot_merlin
+        ~has_dot_merlin:true
 
   let requires t ~loc ~dir ~scope ~dep_kind ~libraries
         ~preprocess ~has_dot_merlin =