sweet: drop tile38 -threads flag

The last use of this flag in tile38 was dropped in
https://github.com/tidwall/tile38/commit/3ae59274e360c4830af514750ec9899bfb11aa98
(1.17.0), which is significantly older than our version (1.29.1).

Change-Id: I6a6a636c6742845cca38cc025b5eab9d8247267f
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/686475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>