Remove unused variables in glass/graph_engine/apps/RandomWalk.h

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.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: dmm-fb

Differential Revision: D56065775

fbshipit-source-id: 77be2f0fdaa133fe6ad5121b0b6c2ee5246bdb32

glean/rts/inventory.cpp

@@ -26,7 +26,6 @@ Inventory::Inventory() : first_id(Pid::lowest())
 {}
 
 Inventory::Inventory(std::vector<Predicate> ps) {
-  size_t n;
   first_id = Pid::invalid();
   auto last_id = Pid::lowest();
   for (const auto& p : ps) {