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>

fs/btrfs/fs.h

@@ -50,18 +50,8 @@ struct btrfs_subpage_info;
 struct btrfs_stripe_hash_table;
 struct btrfs_space_info;
 
-/*
- * Minimum data and metadata block size.
- *
- * Normally it's 4K, but for testing subpage block size on 4K page systems, we
- * allow DEBUG builds to accept 2K page size.
- */
-#ifdef CONFIG_BTRFS_DEBUG
-#define BTRFS_MIN_BLOCKSIZE	(SZ_2K)
-#else
+/* Minimum data and metadata block size. */
 #define BTRFS_MIN_BLOCKSIZE	(SZ_4K)
-#endif
-
 #define BTRFS_MAX_BLOCKSIZE	(SZ_64K)
 
 #define BTRFS_MAX_EXTENT_SIZE SZ_128M