chore(3p/nix): Remove builtins.exec

This is a bad idea, it shouldn't exist, nixpkgs doesn't use it.

Change-Id: Ic4d1b936d8f059d5c40f0567af165b02427d7e36
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1241
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/primops.hh

@@ -20,7 +20,5 @@ struct RegisterPrimOp {
 /* Load a ValueInitializer from a DSO and return whatever it initializes */
 void prim_importNative(EvalState& state, const Pos& pos, Value** args,
                        Value& v);
-/* Execute a program and parse its output */
-void prim_exec(EvalState& state, const Pos& pos, Value** args, Value& v);
 
 }  // namespace nix