rustPlatform.fetchCargoTarball: remove explicit sparse protocol

sparse protocol is now the default since rust 1.70

(cherry picked from commit f4a6750)

Author: figsoda

pkgs/build-support/rust/fetch-cargo-tarball/default.nix

@@ -63,10 +63,6 @@ in stdenv.mkDerivation ({
     export CARGO_HOME=$(mktemp -d cargo-home.XXX)
     CARGO_CONFIG=$(mktemp cargo-config.XXXX)
 
-    # https://blog.rust-lang.org/2023/03/09/Rust-1.68.0.html#cargos-sparse-protocol
-    # planned to become the default in 1.70
-    export CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse
-
     if [[ -n "$NIX_CRATES_INDEX" ]]; then
     cat >$CARGO_HOME/config.toml <<EOF
     [source.crates-io]