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_docmd.h

@@ -25,7 +25,6 @@ typedef struct {
   int save_restart_edit;
   bool save_msg_didout;
   int save_State;
-  int save_insertmode;
   bool save_finish_op;
   long save_opcount;
   int save_reg_executing;