docs: drop stale "service required" warnings from language guides

[detail-app]

The 33 per-language guides still claimed a Dockerfile.miren alone needs an
explicit service and that deploy stops with no services defined otherwise.
This was made false by first-class image support: with no service declared,
Miren now synthesizes a web service from the image's ENTRYPOINT+CMD, so
deploy succeeds rather than failing.

A developer following the Dart guide, for example, would expect
miren deploy of a Dockerfile.miren with no Procfile to fail — when it now
runs the image's startup command directly, contradicting the guide's
"Deploying without a service fails" note.

Introduced by commit bb15676 (@phinze, #1108)

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[evanphx]

Premise verified: no services defined no longer exists anywhere in the Go source, and buildServicesConfig in servers/build/build.go synthesizes a web service whenever none is declared and web = false has not vetoed it. The claim these 33 guides were making is dead, so removing it is right.

The earlier test-blackbox-pop failure was infrastructure, not this change (starting buildkit: task miren-buildkit: already exists on a docs-only diff). It passes on a re-run.

One idea for a follow-up, deliberately not folded in here since it is new content rather than a stale-claim removal: several guides now read "Add a Procfile:" with no rationale, which still leaves the impression that it is required. A clause saying the image ENTRYPOINT/CMD is the fallback would make the option visible.