Remove "Setting GOROOT" custom install instruction

[JoeCortopassi]

doc: Setting GOROOT is no longer necessary for custom installation as of 1.10 (https://go-review.googlesource.com/c/go/+/42533)

Fixes #25002

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Uploaded patch set 2: Commit message was updated.

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Uploaded patch set 3: Commit message was updated.

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Message from Giovanni Bajo:

Patch Set 3:

Please, read https://tip.golang.org/doc/contribute.html#commit_messages on how to write commit messages. You will have to edit the GitHub PR description to change it (have a look at how it appears in Gerrit).

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Message from Brad Fitzpatrick:

Patch Set 3:

https://github.com/golang/go/wiki/CommitMessage also has good docs on this.

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[agnivade]

You should abandon golang.org/cl/109955.