Updated `release-guide.md`.
Updated `release-guide.md`.

Removed unnecessary steps and added the steps for pushing changes to
remote master and release branch.

Review: https://reviews.apache.org/r/71415

Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,abudnik/mesos