src: relax handling of write wrap during cleanup

[addaleax]

One of the HTTP2 tests that are being updated upstream in Node.js leaves around a WriteWrap instance that is not passed over to libuv and therefore can’t be cleaned up.

Address that issue by properly implementing delete self; for WriteWraps using the existing Dispose().

Checklist

• [x] make -j4 test (UNIX), or vcbuild test (Windows) passes
• [x] commit message follows commit guidelines

Affected core subsystem(s)

src

[addaleax]

Landed in 1d01ddbe1e5bdf48a2e866d1bac307a6e7a24e96