offboard irbe from the cert-manager maintainers

Irbe stepped down and joined the cert-manager Emeriti Maintainers.

Ref: cert-manager/community#31

Author: maelvls

communication/slack-config/usergroups.yaml

@@ -315,7 +315,6 @@ usergroups:
       - wallrj
       - jakexks
       - maelvls
-      - irbekrm
       - sgtcodfish
       - inteon
       - ThatsMrTalbot

communication/slack-config/users.yaml

@@ -99,7 +99,6 @@ users:
   ilya-zuyev: U018UF2G42F
   imran: U03K4862X7U
   inteon: U03RG8315NW
-  irbekrm: UTX3WBNGZ
   IsaacPD: U01B0C1R9NZ
   jakexks: U0AKBM9EC
   JamesLaverack: U9MK7274Y