node: remove idle gc

Remove the idle garbage collector. Its purpose was to run the garbage collector
when the application is idle but it never worked quite right. Many people have
complained over the years that with heaps > 128 MB, a node.js process never
sleeps anymore; instead, it spends nearly 100% of its CPU time trying to
collect garbage.

Back in the old days, idle GC probably was a good idea. But with V8's current
incremental collector, idle gc appears to offer no time or space benefits
whatsoever and indeed seems actively harmful. Remove it.

Fixes #3870.