refactor!: delete insertmode (#18547)

Neovim already removed `evim` (or any similar flags). The 'insertmode'
option is a weird remnant, so get rid of it.

The 'insertmode' option is replaced with a script that closely emulates
the option. This script is documented at :help 'insertmode'

src/nvim/ex_cmds.c

@@ -2880,10 +2880,6 @@ int do_ecmd(int fnum, char *ffname, char *sfname, exarg_T *eap, linenr_T newlnum
     redraw_curbuf_later(NOT_VALID);     // redraw this buffer later
   }
 
-  if (p_im && (State & MODE_INSERT) == 0) {
-    need_start_insertmode = true;
-  }
-
   // Change directories when the 'acd' option is set.
   do_autochdir();
 
@@ -4909,9 +4905,8 @@ void ex_help(exarg_T *eap)
     }
   }
 
-  if (!p_im) {
-    restart_edit = 0;               // don't want insert mode in help file
-  }
+  restart_edit = 0;               // don't want insert mode in help file
+
   // Restore KeyTyped, setting 'filetype=help' may reset it.
   // It is needed for do_tag top open folds under the cursor.
   KeyTyped = old_KeyTyped;