jbd2: drop jbd2_fc_init documentation

Now that jbd2_fc_init is dropped, drop its docs too.

Signed-off-by: Harshad Shirwadkar <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

Documentation/filesystems/journalling.rst

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 JBD2 to also allows you to perform file-system specific delta commits known as
-fast commits. In order to use fast commits, you first need to call
-:c:func:`jbd2_fc_init` and tell how many blocks at the end of journal
-area should be reserved for fast commits. Along with that, you will also need
-to set following callbacks that perform correspodning work:
+fast commits. In order to use fast commits, you will need to set following
+callbacks that perform correspodning work:
 
 `journal->j_fc_cleanup_cb`: Cleanup function called after every full commit and
 fast commit.