Revert "runtime: scavenge memory upon allocating from scavenged memory"

This reverts commit 8e093e7
(CL 159500).

Reason for revert: Increases memory allocation latency in certain
situations.

Fixes #31679.

Change-Id: I15e02c53a58009fd907b619b8649de2cdeb29ef0
Reviewed-on: https://go-review.googlesource.com/c/go/+/174102
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

src/runtime/mheap.go

@@ -1198,16 +1198,6 @@ HaveSpan:
 		// heap_released since we already did so earlier.
 		sysUsed(unsafe.Pointer(s.base()), s.npages<<_PageShift)
 		s.scavenged = false
-
-		// Since we allocated out of a scavenged span, we just
-		// grew the RSS. Mitigate this by scavenging enough free
-		// space to make up for it.
-		//
-		// Also, scavengeLargest may cause coalescing, so prevent
-		// coalescing with s by temporarily changing its state.
-		s.state = mSpanManual
-		h.scavengeLargest(s.npages * pageSize)
-		s.state = mSpanFree
 	}
 	s.unusedsince = 0