Remove CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse in CI (#3196)

This should have been done as part of #2766.

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tools/ci-build/Dockerfile

@@ -190,15 +190,13 @@ COPY --chown=build:build --from=cargo_mdbook /opt/cargo/bin/mdbook /opt/cargo/bi
 COPY --chown=build:build --from=cargo_mdbook_mermaid /opt/cargo/bin/mdbook-mermaid /opt/cargo/bin/mdbook-mermaid
 COPY --chown=build:build --from=musl_toolchain /usr/local/musl/ /usr/local/musl/
 ENV PATH=$PATH:/usr/local/musl/bin/
-# TODO(Rust 1.70): The sparse registry config (`CARGO_REGISTRIES_CRATES_IO_PROTOCOL`) can be removed when upgrading to Rust 1.70
 ENV PATH=/opt/cargo/bin:$PATH \
     CARGO_HOME=/opt/cargo \
     RUSTUP_HOME=/opt/rustup \
     JAVA_HOME=/usr/lib/jvm/java-11-amazon-corretto.x86_64 \
     GRADLE_USER_HOME=/home/build/.gradle \
     RUST_STABLE_VERSION=${rust_stable_version} \
     RUST_NIGHTLY_VERSION=${rust_nightly_version} \
-    CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse \
     CARGO_INCREMENTAL=0 \
     RUSTDOCFLAGS="-D warnings" \
     RUSTFLAGS="-D warnings" \