Revert "nvim: plugin: Add a simple whitespace plugin"

This reverts commit 891fae907b.

Now that we have mini.trailspace enabled by default, drop this
whitespace plugin.

nvim/.config/nvim/plugin/whitespace.vim

@@ -1,12 +0,0 @@
-" https://vi.stackexchange.com/questions/454/whats-the-simplest-way-to-strip-trailing-whitespace-from-all-lines-in-a-file
-function! Preserve(command)
-  " Preparation: save window state
-  let l:saved_winview = winsaveview()
-  " Run the command:
-  execute a:command
-  " Clean up: restore previous window position
-  call winrestview(l:saved_winview)
-endfunction
-
-nnoremap <unique> <silent> <Leader>ws  :call Preserve("%s/\\s\\+$//e")<CR>
-xnoremap <unique> <silent> <Leader>ws  :call Preserve("%s/\\s\\+$//e")<CR>