chore: remove stale superpowers plans and designs

Three agent-oriented plan/spec docs were added three weeks ago and never
acted on — every one has 0 checkboxes ticked and no matching commits:

- plans/2026-04-11-remove-sqlite-rembed-and-rust.md       (0/52)
- plans/2026-04-12-infra-dbdeployer-mysql57.md            (0/29)
- specs/2026-04-12-infra-dbdeployer-mysql57-design.md

sqlite-rembed and the Rust toolchain are still in place, and
infra-mysql57 has not been replaced by dbdeployer. If this work is still
wanted, file fresh issues; carrying stale plan checklists in the tree
misleads future agent-driven work.

Author: renecannao