chore: drop GMP from the build instructions

Once stage 0 is updated, no stage uses GMP or the `USE_GMP`/`FORCE_GMP` options anymore. The build docs, the Gitpod image and the notes on stale Nix store paths no longer mention GMP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
7 files changed