Revert "Restore parent mount namespace before executing a child process"

This reverts commit a0ef21262f. This
doesn't work in 'nix run' and nix-shell because setns() fails in
multithreaded programs, and Boehm GC mark threads are uncancellable.

Fixes #2646.

src/nix/repl.cc

@@ -337,8 +337,6 @@ static int runProgram(const string & program, const Strings & args)
     if (pid == -1) throw SysError("forking");
     if (pid == 0) {
         restoreAffinity();
-        restoreSignals();
-        restoreMountNamespace();
         execvp(program.c_str(), stringsToCharPtrs(args2).data());
         _exit(1);
     }