chore(users/Profpatsch): replace Label with pa-label

We upstreamed some of these modules to hackage, so I can get rid of it
here.

Change-Id: I70e1e864a81029cadbbd96cc019a768728431cff
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8659
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI
Autosubmit: Profpatsch <mail@profpatsch.de>
Reviewed-by: sterni <sternenseemann@systemli.org>

users/Profpatsch/mailbox-org/mailbox-org.cabal

@@ -10,6 +10,7 @@ executable mailbox-org
     build-depends:
         base >=4.15 && <5,
         my-prelude,
+        pa-label,
         exec-helpers,
         netencode,
         text,