go: remove misleading comment in go.env

[carlocab]

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See #202030 and golang/go#70949.

In particular, our current go.env contains

# Automatically download newer toolchains as directed by go.mod files.
# See https://go.dev/doc/toolchain for details.
GOTOOLCHAIN=local

The comment about automatically downloading newer toolchains applies
only whenever GOTOOLCHAIN=auto, but we change that to
GOTOOLCHAIN=local. Therefore, let's get rid of the comment that no
longer applies after our GOTOOLCHAIN change.

[github-actions]

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