writev: retract the topic until we have a better emulation

The emulation layer we added for writev(3p) tries to be too faithful
to the spec that on systems with SSIZE_MAX set to lower than 64kB to
fit a single sideband packet would fail just like the real system
writev(), which makes our use of writev() for sideband messages
unworkable.

Let's revert them and reboot the effort after the release.  The
reverted commits are:

    $ git log -Swritev --oneline 8023abc632^..v2.52.0-rc1
    89152af176 cmake: use writev(3p) wrapper as needed
    26986f4cba sideband: use writev(3p) to send pktlines
    1970fcef93 wrapper: introduce writev(3p) wrappers
    3b9b2c2a29 compat/posix: introduce writev(3p) wrapper

8023abc632 is the merge of ps/upload-pack-buffer-more-writes topic to
the mainline.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>