main.star: remove sizespecializedmalloc builders

The experiment is now the default and the experiment will be removed in
Go 1.28.

For #80279

Change-Id: Ia9609de82ae184da838acba679490ee16a6a6964
Reviewed-on: https://go-review.googlesource.com/c/build/+/807000
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>