Remove unused variables in glean/rts/tests/SubstitutionTest.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: palmje

Differential Revision: D56022388

fbshipit-source-id: 1ce16bb29d0a9b4f5735122089f8e9c51846a27f

glean/rts/tests/SubstitutionTest.cpp

@@ -36,7 +36,6 @@ struct Arbitrary<Substitution> {
 template <>
 struct Arbitrary<Intervals> {
   static Gen<Intervals> arbitrary() {
-    const auto l = Id::lowest().toWord();
     return gen::mapcat(
         gen::arbitrary<Substitution>(),
         [](const Substitution& subst) -> Gen<Intervals> {

glean/tools/diff/lib.cpp

@@ -379,7 +379,6 @@ const char *glean_diff(
   folly::Future<folly::Unit> read_db = folly::via(executor, [&](){
     std::atomic<int> next_batch = 0;
     std::list<folly::Future<Group>> read_queue {};
-    bool finished_reading = false;
 
     for (auto i = 0; i < parallel_reads; i++) {
       read_queue.push_back(folly::via(executor, [&](){