btrfs: remove 2K block size support

Originally 2K block size support was introduced to test subpage (block
size < page size) on x86_64 where the page size is exactly the original
minimal block size.

However that 2K block size support has some problems:

- No 2K nodesize support
  This is critical, as there is still no way to exercise the subpage
  metadata routine.

- Very easy to test subpage data path now
  With the currently experimental large folio support, it's very easy to
  test the subpage data folio path already, as when a folio larger than
  4K is encountered on x86_64, we will need all the subpage folio states
  and bitmaps.

  So there is no need to use 2K block size just to verify subpage data
  path even on x86_64.

And with the incoming huge folio (2M on x86_64) support, the 2K block
size will easily double the bitmap size, considering the burden to
maintain and the limited extra coverage, I believe it's time to remove
it for the incoming huge folio support.

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

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