jbd2: drop jbd2_fc_init documentation

Now that jbd2_fc_init is dropped, drop its docs too.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-8-harshadshirwadkar@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>