offlineimap: remove optional insecure kerberos dependency

was marked insecure in c8dbbe5
because of apple/ccs-pykerberos#31

(cherry picked from commit 5fec5eb)

Author: ajs124

pkgs/tools/networking/offlineimap/default.nix

@@ -32,7 +32,6 @@ python3.pkgs.buildPythonApplication rec {
     certifi
     distro
     imaplib2
-    kerberos
     pysocks
     rfc6555
     urllib3