promlib: remove experimental schema/SQL-abstraction integration

[kylebrandt]

Winds down the experimental, feature-gated schemads-based schema discovery and SQL→PromQL transpilation/pushdown in promlib (added under the dsAbstractionApp experiment). It was never enabled by default and has no user-facing effect.

Effectively reverts

• promlib: FOR clause table hints → PromQL (rate/step/instant/agg) #33 — promlib: FOR clause table hints → PromQL (rate/step/instant/agg) — introduced the schemads integration (sql.go, resource/schema.go, flatten.go) and the library.go wiring
• promlib: pushdown capabilities and aggregation hint handling #51 — promlib: pushdown capabilities and aggregation hint handling
• Surface per-metric metadata via schemads TableMetadata #79 — Schemads: Surface per-metric metadata via schemads TableMetadata

(Incidental touches to the removed files from non-abstraction PRs are also trimmed but those PRs are not reverted: #93 "decoding compressed responses" keeps its general resource.go/utils work — only its SchemaProvider additions + that one test go away; dependency bumps #69/#198 are superseded by removing the schemads dep.)

Removed

• pkg/promlib/sql.go — normalizeGrafanaSQLRequest, FOR-clause → PromQL, wrapAggregation
• pkg/promlib/resource/schema.go — SchemaProvider (incl. per-metric metadata)
• pkg/promlib/flatten.go — multi-frame → tabular flattening
• schemads wiring in library.go and the TestSchemaProvider case in resource_test.go
• github.com/grafana/schemads dependency (and its transitive deps) from both go.mod modules

Verification

• go build ./... + go test ./... pass in both pkg/promlib and the root module
• No residual references to any abstraction symbol; gofmt/goimports clean

Note: downstream consumers (e.g. grafana/grafana, which pins pkg/promlib) pick this up on the next promlib version bump.

[github-actions]

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