cmd/internal/objabi: make GOEXPERIMENT=none mean "no experiment flags"

CL 307819 made GOEXPERIMENT=none mean "restore baseline experiment
configuration". This is arguably what you want because any deviation
from the baseline configuration is an "experiment". However, cmd/dist
requires this to mean "disable all experiment flags", even if some
flags are enabled in the baseline configuration, because its build
system doesn't know how to deal with any enabled experiment flags.

Hence, make GOEXPERIMENT=none mean "disable all experiment flags"
again.

Change-Id: I1e282177c3f62a55eb9c36566c75672808dae9b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/309010


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>