jbd2: drop declaration of journal_sync_buffer()

The journal_sync_buffer() function was never carried over from jbd to
jbd2.  So get rid of the vestigal declaration of this (non-existent)
function.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>