ftp: Remove duplicate session ID in NDC

Addresses the issue that the session ID is stored twice in the diagnostic
context of log messages generated by the FTP door.

The NDC is already initialized with the session ID in Tranfer#initSession.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/5610/
(cherry picked from commit 6cfd0b63c8671a53592ba007583f1a09dae4e0a2)
Merge pull request #89 from gbehrmann/fix/2.6/rb5610

ftp: Remove duplicate session ID in NDC