fix(docs): drop the self-issued credential note from integration pages

Three review rounds landed on this one sentence, each asking for accuracy
the generated form cannot carry. The models genuinely differ: an Airtable
PAT and a HubSpot private app are limited by scopes chosen at creation, a
Monday token tracks its creator's current permissions rather than a
snapshot, an Attio key uses workspace-admin-selected scopes, a Webflow
token is bound to one site, Salesforce runs as a configured Run As user,
Zoom is a Marketplace app with its own grant, and a Google service account
depends on IAM with delegation only when impersonating. Twenty-odd models
across thirty-four pages is per-provider prose, and inventing it from a
scope table would publish guesses.

Remove the sentence. The scope tables are what this PR is for, and they
stay exactly as they were. The one general statement worth making survives
where it is true and where a reader configuring credentials will meet it:
the self-hosting reference already says each table is the OAuth flow's
request and that a hand-issued credential carries its own access, which
those tables do not govern.

Reverts the catalog's `serviceAccountProviderId` field and the
`SERVICE_ACCOUNT_EXCLUDED_SCOPES` move along with it, since nothing reads
either once the note is gone.

Author: mzxchandra