lib: remove an unnecessary coverage check

Checklist

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

Affected core subsystem(s)

coverage

Fixes: https://github.com/nodejs/node/issues/11445 Refs: f65a48fc

Based on @addaleax's comment here.

Before (coverage.nodejs.org, master)

After

And the specific process._exiting lines that f65a48fc was supposed to hit are indeed hit with this change:

cc @addaleax, @mhdawson @CurryKitten