docs(e2e): drop the link to the never-committed e2e-plan.md

The 'Full plan' section references ../../e2e-plan.md, which has no
commits anywhere in the repository's history (git log --all is empty) —
the file was evidently meant to ship with the suite but never did.

tests/e2e/README.md

@@ -54,7 +54,3 @@ records for debugging).
   upload specs. Body content must be deterministic — no dates, UUIDs, or
   random tokens.
 
-## Full plan
-
-See [`../../e2e-plan.md`](../../e2e-plan.md) for the phased rollout,
-parallelization model, and Tier-A/B/C spec inventory.