Using flet to replace kill-region with delete-region was a hack,
flet is now (GNU Emacs 24.3) deprecated and at least two people
have reported an issue where using go--delete-whole-line would
permanently break their kill ring. While that issue is probably
caused by faulty third party code (possibly prelude), it's easier
to write a clean implementation than to tweak the hack.

LGTM=ruiu, adonovan
R=adonovan, ruiu
CC=adg, golang-codereviews
https://golang.org/cl/106010043