btrfs: remove the alignment check in end_bbio_data_write()

The check is not necessary because:

- There is already assert_bbio_alignment() at btrfs_submit_bbio()

- There is also btrfs_subpage_assert() for all btrfs_folio_*() helpers

- The original commit mentions the check may go away in the future
Commit 17a5adccf3fd01 ("btrfs: do away with non-whole_page extent
I/O") introduced the check first, and in the commit message:

I've replaced the whole_page computations with warnings, just to be
sure that we're not issuing partial page reads or writes. The
warnings should probably just go away some time.

- No similar check in all other endio functions
No matter if it's data read, compressed read or write.

- There is no such report for very long
I do not even remember if there is any such report.

Thus the need to do such check in end_bbio_data_write() is very weak,
and we can just get rid of it.

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>