Fix vim-tmux-focus-events errors in command-line

When changing focus away and back to vim while in the middle of editing
the command-line, vim-tmux-focus-events emits errors from the function
called by the `:cnoremap` mappings. This patch replaces these mappings
with a call to `do#sink()` which does nothing. Simply removing the
mappings will result in `<F24><F25>` being inserted on the command-line
since tmux will send this text when `tmux set-option -g focus-events
on`, this is why `do#sink()` is needed.

autoload/do.vim

@@ -96,3 +96,7 @@ function! do#signify() abort
   " Enable update on text change e.g. undo/redo
   autocmd signify TextChanged <buffer> call sy#start()
 endfunction
+
+" A sink for mappings to do nothing
+function! do#sink() abort
+endfunction