[dev.simd] internal/buildcfg: default-disable simd experiment, for merge w/ master

this will be undone after the two branches are both in sync.

Change-Id: I114e8a9b4ed29646ec22db1135e59f8bad47c4da
Reviewed-on: https://go-review.googlesource.com/c/go/+/726521
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/internal/buildcfg/exp.go

@@ -83,7 +83,6 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	baseline := goexperiment.Flags{
 		RegabiWrappers:        regabiSupported,
 		RegabiArgs:            regabiSupported,
-		SIMD:                  goarch == "amd64", // TODO: remove this (default to false) when dev.simd is merged
 		Dwarf5:                dwarf5Supported,
 		RandomizedHeapBase64:  true,
 		SizeSpecializedMalloc: true,