Commit d7e0ad4b removed the next_gc manipulation from mSpan_Sweep, but
left in the traceNextGC() for recording the updated next_gc
value. Remove this now unnecessary call.

Change-Id: I28e0de071661199be9810d7bdcc81ce50b5a58ae
Reviewed-on: https://go-review.googlesource.com/8894Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>