src: remove keep alive option from SetImmediate()

This is no longer necessary now that the copyable `BaseObjectPtr`
is available (as opposed to the only-movable `v8::Global`).

Backport-PR-URL: https://github.com/nodejs/node/pull/32301
PR-URL: https://github.com/nodejs/node/pull/30374
Refs: https://github.com/nodejs/quic/pull/141
Refs: https://github.com/nodejs/quic/pull/149
Refs: https://github.com/nodejs/quic/pull/141
Refs: https://github.com/nodejs/quic/pull/165


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>