btrfs: remove experimental offload csum mode

btrfs: remove experimental offload csum mode

The offload csum mode was introduced to allow developers to compare the
performance of generating checksum for data writes at different timings:

- During btrfs_submit_chunk()
  This is the most common one, if any of the following condition is met
  we go this path:

  * The csum is fast
    For now it's CRC32C and xxhash.

  * It's a synchronous write

  * Zoned

- Delay the checksum generation to a workqueue

However since commit dd57c78aec39 ("btrfs: introduce
btrfs_bio::async_csum") we no longer need to bother any of them.

As if it's an experimental build, async checksum generation at the
background will be faster anyway.

And if not an experimental build, we won't even have the offload csum
mode support.

Considering the async csum will be the new default, let's remove the
offload csum mode code.

There will be no impact to end users, and offload csum mode is still
under experimental features.

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