Remove unused variables in glean/rts/ownership.cpp

Summary:
LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with `[[maybe_unused]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

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Reviewed By: dmm-fb

Differential Revision: D56065767

fbshipit-source-id: 88646966e562356333affb0c08e34a647ba2b7c8

glean/rts/ownership.cpp

@@ -234,7 +234,6 @@ FOLLY_NOINLINE void completeOwnership(
 
   // Iterate over facts backwards - this ensures that we get all dependencies.
   const auto min_id = lookup.startingId();
-  const auto max_id = lookup.firstFreeId();
   const auto owner = [&](Id id) -> Uset*& {
     if (id < min_id) {
       return sparse[id.toWord()];