mds/Server.cc: remove unused initialization of 'destdnl'

Remove initialization of 'destdnl' since the assigned value was
never used and the same call is used some lines later again before
first usage. Fix for:

6579 CDentry::linkage_t *destdnl = destdn->get_linkage();
     Value stored to 'destdnl' during its initialization is never read

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>