Revert "[dev.simd] internal/buildcfg: disable SIMD experiment by default for merge to master"

This reverts commit 9816f3ec4a1cc6693ebfdf79b8d6213004005f63.

Reason for revert: merge is complete.

Change-Id: I33755453a2234b12cd623fa7bc71920f7a381b31
Reviewed-on: https://go-review.googlesource.com/c/go/+/783240
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: David Chase <drchase@google.com>