timers: make setImmediate() immune to tampering
Make setImmediate() immune to process
global tampering by removing
the dependency on the process._immediateCallback
property.
Fixes: https://github.com/nodejs/node/issues/17681
CI: https://ci.nodejs.org/job/node-test-commit/14914/
CI: https://ci.nodejs.org/job/node-test-commit/14918/