fix(crypto): remove vendored secp256k1 to drop duplicate symbols at link

[nebojsa94]

Why

When tenderly-core links a binary that pulls in both nitro's libstylus.a and other modules that vendor secp256k1 (e.g. zksync-go), the macOS linker errors with ~50 duplicate _secp256k1_* symbols because two different cgo-compiled .o files each embed the full libsecp256k1 implementation:

duplicate symbol '_secp256k1_ec_pubkey_serialize' in:
    /tmp/go-link-…/000003.o
    /tmp/go-link-…/000030.o
… (×51)

On Linux CI this was masked by -Wl,--allow-multiple-definition in tenderly-core's BUILD_FLAGS, but newer macOS ld doesn't accept that flag, so local macOS builds fail outright.

This drops nitro's own vendored libsecp256k1 + Go wrappers under go-ethereum/crypto/secp256k1/ (185 files, ~70K deletions) and switches the two consumer sites (go-ethereum/crypto/signature_cgo.go, tests/fuzzers/secp256k1/secp_test.go, system_tests/timeboost_test.go) to upstream github.com/ethereum/go-ethereum/crypto/secp256k1. MVS in the consumer binary then keeps a single copy.

The Rust side (libstylus.a) is unchanged — only Go imports/files moved.

Tests

• go mod tidy in net-nitro succeeds after the import-path renames; it auto-adds github.com/ethereum/go-ethereum v1.10.26 (MVS will pick the higher tenderly-core pin v1.17.2 at consumer build time).
• The nitro static-library artifacts (nitro-d3ca07605e9b-{linux,macos}.tar.gz) were republished by gsutil cp from d7575265ccc0 — byte-identical Rust output.

Services affected

Any tenderly-core service that links nitro into a binary together with another secp256k1-vendoring module (zksync, etc.). Concretely: tenderly-server, node-server services, anything building from cmd/*.