Fix helm-top-command (#1532)

To make users be able customize this variable

helm-sys.el

@@ -33,13 +33,10 @@
   :group 'helm-sys)
 
 
-(defun helm-top-command-set-fn (var _value)
-  (set var
-       (cl-case system-type
-         (darwin "env COLUMNS=%s ps -axo pid,user,pri,nice,ucomm,tty,start,vsz,%%cpu,%%mem,etime,command")
-         (t      "env COLUMNS=%s top -b -n 1"))))
-
-(defcustom helm-top-command "env COLUMNS=%s top -b -n 1"
+(defcustom helm-top-command
+  (cl-case system-type
+    (darwin "env COLUMNS=%s ps -axo pid,user,pri,nice,ucomm,tty,start,vsz,%%cpu,%%mem,etime,command")
+    (t      "env COLUMNS=%s top -b -n 1"))
   "Top command used to display output of top.
 To use top command, a version supporting batch mode (-b option) is needed.
 On Mac OSX top command doesn't support this, so ps command
@@ -50,8 +47,7 @@ working properly, that is 12 elements with the 2 first being
 PID and USER and the last 4 being %CPU, %MEM, TIME and COMMAND.
 A format string where %s will be replaced with `frame-width'."
   :group 'helm-sys
-  :type 'string
-  :set  'helm-top-command-set-fn)
+  :type 'string)
 
 (defcustom helm-top-poll-delay 1.5
   "Helm top poll after this dealy when `helm-top-poll-mode' is enabled.