Remove the Table of contents from the 4.0.0 upgrade guide

[rileykarson]

The Registry actually adds a navbar on the side already, so I think we can omit the inline TOC: https://registry.terraform.io/providers/hashicorp/google/latest/docs/guides/version_4_upgrade

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 81 deletions(-))
Terraform Beta: Diff ( 1 file changed, 81 deletions(-))

[c2thorn]

Goodbye ToC, the merge conflicts were fun while they lasted