Revert "Use mold linker for development."

This reverts commit 4f1a1bbce5. This was
intended to improve the development speed, but it breaks the build. This
is because even when cross-compiling, the Rust compiler does compile
some code for the host target for code-generation.

.cargo/config.toml

@@ -1,7 +1,3 @@
-[target.x86_64-unknown-linux-gnu]
-linker = "clang"
-rustflags = ["-C", "link-arg=-fuse-ld=mold"]
-
 [target.x86_64-unknown-linux-musl]
 rustflags = ["-C", "target-feature=+crt-static", "-C", "link-arg=-s", "-C", "link-arg=-lc", "-C", "link-arg=-lgcc"]
 

shell.nix

@@ -18,7 +18,6 @@ pkgs.mkShell {
     pkgs.protoc-gen-grpc-web    # grpc-web api
     pkgs.protoc-gen-go          # go api
     pkgs.protoc-gen-go-grpc
-    pkgs.mold
   ];
   LIBCLANG_PATH = "${pkgs.llvmPackages.libclang.lib}/lib";
   BINDGEN_EXTRA_CLANG_ARGS = "-I${pkgs.llvmPackages.libclang.lib}/lib/clang/${pkgs.llvmPackages.libclang.version}/include";