signalfd: drop an obsolete comment

Commit fbe38120 ("signalfd: convert to ->read_iter()") removed the
call to anon_inode_getfd() by splitting fd setup into two parts. Drop the
comment referencing the internal details of that function.

Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Link: https://lore.kernel.org/r/20240520090819.76342-2-pchelkin@ispras.ru


Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>