libutil: remove runProgram2 stdin functionality

this was only used in one place, and that place has been rewritten to
use a temporary file instead. keeping this around is not very helpful
at this time, and in any case we'd be better off rewriting subprocess
handling in rust where we not only have a much safer library for such
things but also async frameworks necessary for this easily available.

Change-Id: I6f8641b756857c84ae2602cdf41f74ee7a1fda02

src/libcmd/repl.cc

@@ -242,8 +242,7 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
 {
 }
 
-void runNix(Path program, const Strings & args,
-    const std::optional<std::string> & input = {})
+void runNix(Path program, const Strings & args)
 {
     auto subprocessEnv = getEnv();
     subprocessEnv["NIX_CONFIG"] = globalConfig.toKeyValue();
@@ -252,7 +251,6 @@ void runNix(Path program, const Strings & args,
         .program = settings.nixBinDir+ "/" + program,
         .args = args,
         .environment = subprocessEnv,
-        .input = input,
     });
 
     return;