bcachefs: Delete a spurious assertion

bch_write_op->written used to be a u16, but it's not so the assertion
isn't needed anymore - and 5.1 can send larger bios.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -966,7 +966,6 @@ void bch2_write(struct closure *cl)
 	BUG_ON(!op->nr_replicas);
 	BUG_ON(!op->write_point.v);
 	BUG_ON(!bkey_cmp(op->pos, POS_MAX));
-	BUG_ON(bio_sectors(&op->wbio.bio) > U16_MAX);
 
 	op->start_time = local_clock();