Enable -Wunused-variable in glean/PACKAGE

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it work; test errors are spurious.**

Reviewed By: palmje

Differential Revision: D66847527

fbshipit-source-id: be53693c0d05047e6b0fe28238a1491e1ec7b006

glean/rocksdb/database-impl.cpp

@@ -269,16 +269,15 @@ struct SeekIterator final : rts::FactIterator {
   Fact::Ref get(Demand demand) override {
     if (iter_->Valid()) {
       auto key = input(iter_->key());
-      auto ty = key.fixed<Pid>();
-      assert(ty == type_);
+      assert(key.fixed<Pid>() == type_);
       auto value = input(iter_->value());
       auto id = value.fixed<Id>();
       assert(value.empty());
 
       if (demand == KeyOnly) {
         return Fact::Ref{id, type_, Fact::Clause::fromKey(key.bytes())};
       } else {
-        auto found = db_->lookupById(id, slice_);
+        [[maybe_unused]] auto found = db_->lookupById(id, slice_);
         assert(found);
         return decomposeFact(id, slice_);
       }

glean/rts/stacked.h

@@ -160,12 +160,11 @@ struct Stacked<Define> final : StackedBase<Define> {
         //       similar). This means that there is no need to check against the
         //       value stored in the database as that must be empty, too.
         if (clause.value_size != 0) {
-          bool found = base->factById(id, [&](auto, auto found) {
+          assert(base->factById(id, [&](auto, auto found) {
             if (clause.value() != found.value()) {
               id = Id::invalid();
             }
-          });
-          assert(found);
+          }));
         }
         return id;
       }