Revert "seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_str()"

This reverts commit 6920e362 which is
commit dcc4e572 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: I734f2293bcd140fc75697e27a3d3ccd3940ef845
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>