Remove useless Reline::Key.new and update wrong comment for alt+d (#963)

lib/irb/input-method.rb

@@ -328,10 +328,11 @@ def show_doc_dialog_proc
       ->() {
         dialog.trap_key = nil
         alt_d = [
-          [Reline::Key.new(nil, 0xE4, true)], # Normal Alt+d.
           [27, 100], # Normal Alt+d when convert-meta isn't used.
-          [195, 164], # The "ä" that appears when Alt+d is pressed on xterm.
-          [226, 136, 130] # The "∂" that appears when Alt+d in pressed on iTerm2.
+          # When option/alt is not configured as a meta key in terminal emulator,
+          # option/alt + d will send a unicode character depend on OS keyboard setting.
+          [195, 164], # "ä" in somewhere (FIXME: environment information is unknown).
+          [226, 136, 130] # "∂" Alt+d on Mac keyboard.
         ]
 
         if just_cursor_moving and completion_journey_data.nil?