remove leftover mysql apt-key (#33708) (#33711)

(cherry picked from commit 527ea8421bc5f366209f21c609f200c3f38768c1)

Co-authored-by: sabril <5334504+saturninoabril@users.noreply.github.com>

server/build/docker-preview/Dockerfile

@@ -2,7 +2,6 @@
 # See License.txt for license information.
 FROM postgres:13
 
-RUN apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 467B942D3A79BD29
 RUN apt-get update && apt-get install -y ca-certificates
 
 ARG MATTERMOST_VERSION