classes/core-image: remove SSHSERVER_IMAGE_FEATURES

classes/core-image: remove SSHSERVER_IMAGE_FEATURES

This variable was added in the very same commit that added a mechanism
that makes it obsolete: if you have dropbear and you want openssh, you
just add ssh-server-openssh to your IMAGE_FEATURES and it will replace
dropbear via IMAGE_FEATURES_REPLACES_.

(From OE-Core rev: 6c78d99cc2e54a99af22357d7567f64e864949b9)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>