chore: get rid of dead code and unused variables where appropriate

Looks like some cruft has been left over from previous refactorings.
This removes dead variables, which should not have side effects in their
constructors. In cases where the variable initialization has a purpose
[[maybe_unused]] is inserted to silence compiler warnings.

src/libstore/build/derivation-goal.cc

@@ -1229,7 +1229,7 @@ HookReply DerivationGoal::tryBuildHook()
     hook->toHook.writeSide.close();
 
     /* Create the log file and pipe. */
-    Path logFile = openLogFile();
+    [[maybe_unused]] Path logFile = openLogFile();
 
     std::set<MuxablePipePollState::CommChannel> fds;
     fds.insert(hook->fromHook.readSide.get());

src/libstore/export-import.cc

@@ -13,14 +13,9 @@ void Store::exportPaths(const StorePathSet & paths, Sink & sink)
     auto sorted = topoSortPaths(paths);
     std::reverse(sorted.begin(), sorted.end());
 
-    std::string doneLabel("paths exported");
-    //logger->incExpected(doneLabel, sorted.size());
-
     for (auto & path : sorted) {
-        //Activity act(*logger, lvlInfo, "exporting path '%s'", path);
         sink << 1;
         exportPath(path, sink);
-        //logger->incProgress(doneLabel);
     }
 
     sink << 0;

src/libstore/unix/build/local-derivation-goal.cc

@@ -883,7 +883,7 @@ void LocalDerivationGoal::startBuilder()
         printMsg(lvlVomit, "setting builder env variable '%1%'='%2%'", i.first, i.second);
 
     /* Create the log file. */
-    Path logFile = openLogFile();
+    [[maybe_unused]] Path logFile = openLogFile();
 
     /* Create a pseudoterminal to get the output of the builder. */
     builderOut = posix_openpt(O_RDWR | O_NOCTTY);