process: remove undocumented `now` argument from emitWarning()

process.emitWarning() "now" option is undocumented and a Boolean trap. Remove it before people start adopting it.

We only need it in one place internally. Replace it with an internal-only emitWarningSync() function.

Checklist

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