Goblin: retire Nym from the public toolchains + drop internal test harnesses

- CI (release.yml/build.yml): remove fetch-nym + AWS_LC_SYS_PREBUILT_NASM;
  the default cargo build --release is Tor/arti only now (nip44 fetch kept).
- Cargo.toml: nym-sdk is optional behind the `nym` feature so the default
  build never pulls it.
- Delete the dead Nym-era probes (xrelay_smoke, connect_timing,
  tunnel_measure) that referenced the removed Nym API and broke `cargo test`.
- Untrack the internal E2E harnesses (e2e.rs, nostr_e2e.rs) via .gitignore;
  gate `mod e2e` behind the `e2e-internal` feature so clones still compile.
- Refresh stale Nym comments in build.rs and the build scripts.

.github/actions/fetch-nym/action.yml

@@ -1,23 +0,0 @@
-name: Fetch patched nym SDK
-description: >
-  Clone the patched nym workspace from our own mirror
-  (git.us-ea.st/GRIN/nym, branch `goblin` = upstream nymtech/nym @ b6eb391 +
-  Goblin's Android webpki-roots patch) into ../nym, so the
-  `nym-sdk = { path = "../nym/sdk/rust/nym-sdk" }` dependency resolves.
-  Self-hosted: no upstream-GitHub fetch and no patch-apply step.
-
-runs:
-  using: composite
-  steps:
-    - name: Clone patched nym
-      shell: bash
-      run: |
-        set -euo pipefail
-        DEST="$(dirname "$GITHUB_WORKSPACE")/nym"
-        if [ -e "$DEST/sdk/rust/nym-sdk/Cargo.toml" ]; then
-          echo "nym already present at $DEST"
-          exit 0
-        fi
-        rm -rf "$DEST"
-        git clone --branch goblin --depth 1 https://git.us-ea.st/GRIN/nym.git "$DEST"
-        echo "nym cloned from GRIN/nym@goblin -> $DEST"