btrfs: remove the subpage related warning message

Since the initial enablement of block size < page size support for
btrfs in v5.15, we have hit several milestones for block size < page
size (subpage) support:

- RAID56 subpage support
  In v5.19

- Refactored scrub support to support subpage better
  In v6.4

- Block perfect (previously requires page aligned ranges) compressed write
  In v6.13

- Various error handling fixes involving subpage
  In v6.14

Finally the only missing feature is the pretty simple and harmless
inlined data extent creation, just added in previous patches.

Now btrfs has all of its features ready for both regular and subpage
cases, there is no reason to output a warning about the experimental
subpage support, and we can finally remove it now.

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