content/en/case-studies: remove OWNERS file

As a part of cleaning up inactive members (who haven't been active since
the release of v1.11) from OWNERS, this commit removes the OWNERS file
for content/en/case-studies.

content/en/case-studies/OWNERS

@@ -1,10 +0,0 @@
-# See the OWNERS docs at https://go.k8s.io/owners
-
-# Owned by Kubernetes Blog reviewers.
-options:
-  no_parent_owners: false
-reviewers:
-  - alexcontini
-approvers:
-  - alexcontini
-  - sarahkconway