src: remove StreamBase::kFlagHasWritev

Since libuv 1.21.0, pipes on Windows support `writev` on the
libuv side.

This allows for some simplification, and makes the `StreamBase`
API more uniform (multi-buffer `Write()` is always supported now,
including when used by other non-JS consumers like HTTP/2).

PR-URL: https://github.com/nodejs/node/pull/21527


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>