Remove 'restorescreen' option

The relevant code was never actually in Neovim, most likely due to being
unifdef(1)'d out during the initial import.

see `:h hidden-options'

src/nvim/options.lua

@@ -1855,13 +1855,6 @@ return {
       varname='p_report',
       defaults={if_true={vi=2}}
     },
-    {
-      full_name='restorescreen', abbreviation='rs',
-      type='bool', scope={'global'},
-      vi_def=true,
-      enable_if=false,
-      defaults={if_true={vi=true}}
-    },
     {
       full_name='revins', abbreviation='ri',
       type='bool', scope={'global'},