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/main.c

@@ -530,11 +530,6 @@ int main(int argc, char **argv)
   // 'autochdir' has been postponed.
   do_autochdir();
 
-  // start in insert mode
-  if (p_im) {
-    need_start_insertmode = true;
-  }
-
   set_vim_var_nr(VV_VIM_DID_ENTER, 1L);
   apply_autocmds(EVENT_VIMENTER, NULL, NULL, false, curbuf);
   TIME_MSG("VimEnter autocommands");