locks: remove unused posix_block_lock

posix_lock_file() is used to add a blocked lock to Lockd's block, so
posix_block_lock() is no longer needed.

Signed-off-by: Andy Adamson <andros@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>