chore: remove extensionless MAINTAINERS duplicate (#61)

Removes the extensionless `MAINTAINERS`, superseded by
`MAINTAINERS.adoc`.

**This isn't only a duplicate — the removed file was wrong.** It
carried:
- **no SPDX header**
- maintainership attributed to *"Metadatastician / @metadatastician"*

while `MAINTAINERS.adoc` carries the `CC-BY-SA-4.0` header and correctly
names **Jonathan D.A. Jewell as Lead Maintainer**. It is scaffold
boilerplate — byte-identical across 37 of 40 repos that had it, the
other 3 trivial variants.

Owner-directed 2026-08-26. Raised as a PR because this repo's ruleset
requires pull requests and verified signatures, which a direct API
commit cannot satisfy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath