process: small improvements to internals

Some small tweaks:

1. Remove unnecessary call to a binding which is already guaranteed to be loaded via requiring child_process (and that's even if calling that binding was necessary in the first place)

2. Cleanup setupSignalHandlers, including removing an outdated comment, using null prototype Object, not calling C++ repeatedly to get the binding, etc.

Checklist

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

Affected core subsystem(s)

process