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/options.lua

@@ -1250,9 +1250,9 @@ return {
     },
     {
       full_name='insertmode', abbreviation='im',
-      short_desc=N_("start the edit of a file in Insert mode"),
+      short_desc=N_("No description"),
       type='bool', scope={'global'},
-      varname='p_im',
+      varname='p_force_off',
       defaults={if_true=false}
     },
     {