Remove :open command

From the documentation itself:

  :[range]o[pen]                 Works like |:visual|: end Ex mode.
                                 {Vi: start editing in open mode}
  ...

  Vim does not support open mode, since it's not really useful.  For
  those situations where ":open" would start open mode Vim will leave Ex
  mode, which allows executing the same commands, but updates the whole
  screen instead of only one line.

Part of the reason behind this is to make removing vi_diff.txt easier,
although it's also because :open is not too useful.

Helped-by: @fdinoff
Helped-by: @dsummersl
Helped-by: @mhinz
Helped-by: @justinmk

src/nvim/ex_cmds.lua

@@ -1753,12 +1753,6 @@ return {
     addr_type=ADDR_LINES,
     func='ex_menu',
   },
-  {
-    command='open',
-    flags=bit.bor(RANGE, BANG, EXTRA),
-    addr_type=ADDR_LINES,
-    func='ex_open',
-  },
   {
     command='oldfiles',
     flags=bit.bor(BANG, TRLBAR, SBOXOK, CMDWIN),