Revert "seq_buf: Make DECLARE_SEQ_BUF() usable"

This reverts commit c3b5a7d6 which is
commit 7a8e9cdf upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Iff9677269520e1875d8a041ac47a1ebda13b096f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>