timers: do not expose .unref()._handle._list

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

timers

Description of change

The list is defunct at this point. I believe this to be in effect a minor defect/oversight from 3eecdf9f

Tangentially effects/related to https://github.com/nodejs/node/pull/8372

Edit: cc @indutny?