[Refactor] Remove the unused promql/sql/metricsql frontend features of asap-planner #1065

Example

asap-planner has three frontend features: promql, sql and metricsql, all on by default. They exist so that a runtime planning PromQL alone can build with default-features = false, features = ["promql"] and avoid linking DataFusion and the MetricsQL parser. If a workload's language is disabled, planning fails with UserInputError::FrontendDisabled.

Nothing actually builds that way:

• No crate in the workspace disables them. integration-tests asks for promql + sql, and metricsql stays on through defaults.
• The only consumer is the CI step rust.yml:143-144 (--no-default-features --features promql). Its test half has never exercised the PromQL-only build: the self dev-dependency asap-planner = { path = ".", features = ["test-support"] } turns default features back on (ci: the PromQL-only asap-planner test step still builds with sql and metricsql #1056).
• The sql-only and metricsql-off combinations are never built.

Concrete maintenance problem

The features cost:

• About 18 #[cfg(feature = ...)] sites in crates/planner/src/lib.rs
• cfg-gated tests in crates/planner/tests/{e2e_plan,summary_sharing,sql_table_scans}.rs
• The FrontendDisabled error variant
• A CI step

They protect a build configuration that has no user.

Behavior that must remain unchanged

All three frontends are always available. Planning results are unchanged.

Before and after structure

Before: asap-frontend-{promql,sql,metricsql} are optional dependencies of asap-planner, behind the promql/sql/metricsql features.

After:

• They become regular dependencies.
• The promql/sql/metricsql features, their cfgs, UserInputError::FrontendDisabled and the not(sql) test (e2e_plan.rs:337) are removed.
• The CI step "Check and test asap-planner with the PromQL frontend alone" is removed.
• The test-support feature stays.

The fix for #1056 (unmerged on the ci-speed branch, commit 73eb80e) becomes unnecessary.

Why the proposed structure is minimally complex

The change only deletes code. If a PromQL-only runtime ever needs this, the features can be added back then.

Existing unit and end-to-end protection

• cargo test --workspace
• Corpus snapshots in crates/devtools/tests/corpus_snapshots/; these must stay unchanged.

Compatibility or migration impact

• UserInputError::FrontendDisabled is removed from the public API.
• External users that pass default-features = false or name these features would fail to resolve. ASAPQuery-backend uses asap_planner; confirm before merging that it does not set either.