src: remove PumpMessageLoop() from Start() (?)

[Do NOT merge]

This appears to do nothing, and no existing tests catch any relating behavior on my machine, so... remove it?

Does anyone know in more detail what this does or why we'd need it? @nodejs/v8

CI: https://ci.nodejs.org/job/node-test-pull-request/7182/

Checklist

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

Affected core subsystem(s)

src