runtime: remove darwin-amd64-10_14 builder check

runtime: remove darwin-amd64-10_14 builder check

By now macOS 11 is the minimum required version.

Updates #43926

Change-Id: I13716e2e521c26bd9997f0ea1b717cf72ab7e47c
Reviewed-on: https://go-review.googlesource.com/c/go/+/601276
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>