Remove unused variables in velox/expression/CastExpr-inl.h (#8773)

Summary:
Pull Request resolved: #8773

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: D53779591

fbshipit-source-id: 67b39c9f26a6cd6f783e7d621551cbfe158fe434

velox/expression/CastExpr-inl.h

@@ -717,8 +717,6 @@ void CastExpr::applyCastPrimitives(
   auto* resultFlatVector = result->as<FlatVector<To>>();
   auto* inputSimpleVector = input.as<SimpleVector<From>>();
 
-  auto& resultType = resultFlatVector->type();
-
   if (!hooks_->truncate()) {
     if (!hooks_->legacy()) {
       applyToSelectedNoThrowLocal(context, rows, result, [&](int row) {