bench: remove manual GC from garbage benchmark
Initially the garbage benchmark was flaky because
some runs trigger 3 GCs while others trigger 4 GCs.
This caused 33% flakes.
Then I've added the manual GC triggering that emulates
GOGC=100 but more stable. This fixed the flakiness.
Then we've changed runtime.GC so that it executes 2 GCs.
This increased GC time by 50%, and numbers between releases
become uncomparable.
I don't have a better idea than to remove the manual GC triggerring.

CC=golang-codereviews, rsc
3 files changed