flake.nix: Remove cross glibc

On darwin in `nativeBuildInputs`

```
error: Package _glibc-nolibgcc-2.40-36_ in /nix/store/813bk6hvhfjjjmzlk2cxgxgd25yqq5f8-source/pkgs/development/libraries/glibc
/default.nix:214 is not available on the requested hostPlatform:
         hostPlatform.config = "aarch64-apple-darwin"
```

in `buildInputs`

```
is not available on the requested hostPlatform:
  hostPlatform.config = "riscv64-none-elf"
```

flake.nix

@@ -60,7 +60,6 @@
                         qemu
                         riscv64-linux.gcc
                         riscv64-linux.binutils
-                        riscv64-linux.glibc
                       ];
                       shellHook = ''
                         export CROSS_PREFIX="riscv64-none-elf"