Emacs: Remove README warning of old whitespace bug

The incompatibility of rust-mode with global-whitespace-mode warned
about in the README was actually fixed by commit 581b3db3b3.  Remove the
warning from the README and close #3994.
auto merge of #12663 : MicahChalmer/rust/emacs-remove-ws-bug-warning, r=brson

The incompatibility of rust-mode with global-whitespace-mode warned
about in the README was actually fixed by commit 581b3db3b3.  Remove the
warning from the README and close #3994.