Remove rkt

https://github.com/rkt/rkt is an ex-project.
rkt has kicked the bucket, shuffled off its mortal coil, run down the
curtain and joined the choir invisible.

content/en/docs/reference/glossary/rkt.md

@@ -1,18 +0,0 @@
----
-title: rkt
-id: rkt
-date: 2019-01-24
-full_link: https://coreos.com/rkt/
-short_description: >
-  A security-minded, standards-based container engine.
-
-aka: 
-tags:
-- security
-- tool
----
- A security-minded, standards-based container engine.
-
-<!--more--> 
-
-rkt is an application {{< glossary_tooltip text="container" term_id="container" >}} engine featuring a {{< glossary_tooltip text="Pod" term_id="pod" >}}-native approach, a pluggable execution environment, and a well-defined surface area. rkt allows users to apply different configurations at both the Pod and application level. Each Pod executes directly in the classic Unix process model, in a self-contained, isolated environment.