* lisp/eshell/em-unix.el (eshell/diff): Adjust for diff-mode; do not
set 'compilation-finish-functions', as it is never used. (Bug#65245)
(eshell-diff-quit): Make into an obsolete function alias for 'ignore';
the command has been a no-op for close to two decades.
(eshell-diff-window-config): Make obsolete.