btrfs: remove duplicate btrfs_clear_buffer_dirty() prototype from disk-io.h

The prototype for btrfs_clear_buffer_dirty() is declared in both disk-io.h
and extent_io.h, but the function is defined at extent_io.c. So remove the
prototype declaration from disk-io.h.

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