Remove multirepo PR research temp doc

- Delete greenlight/temp_docs/greenlight-multirepo-pr-research.md (307 lines)

Scratch research notes on scaling greenlight's open-PR fetch from
single-repo REST to multi-repo GraphQL. Findings were exploratory and
not needed in the tree; the current single-repo path is unchanged.

Signed-off-by: Jean Schmidt <contato@jschmidt.me>

Author: jeanschmidt