runtime: remove dedicated scavenger thread

A whole thread is too much for background scavenger that sleeps all the time anyway.
We already have sysmon thread that can do this work.
Also remove g->isbackground and simplify enter/exitsyscall.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr, rlh
https://golang.org/cl/108640043