cmd/compile: drop "buildcfg" from no instrument packages
Package buildcfg was added to this list by CL 403851, but package
buildcfg does not exist.
This was probably intended to refer to internal/buildcfg, but
internal/buildcfg is only used by the compiler so it is not clear why it
couldn't be instrumented.
For #44853.
Change-Id: Iad2517358be79c3eabf240376156bcff0c4bcefc
Reviewed-on: https://go-review.googlesource.com/c/go/+/414516 Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>