Remove unused variables in velox/exec/ContainerRowSerde.cpp (facebook…

…incubator#8785)

Summary:
Pull Request resolved: facebookincubator#8785

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: D53779443

fbshipit-source-id: e9fa9ae3f8fc029064b0515216b82ff02a7ef1fc

velox/exec/ContainerRowSerde.cpp

@@ -167,8 +167,6 @@ void serializeOne<TypeKind::MAP>(
     ByteOutputStream& out) {
   auto map = vector.wrappedVector()->asUnchecked<MapVector>();
   auto wrappedIndex = vector.wrappedIndex(index);
-  auto size = map->sizeAt(wrappedIndex);
-  auto offset = map->offsetAt(wrappedIndex);
   auto indices = map->sortedKeyIndices(wrappedIndex);
   serializeArray(*map->mapKeys(), indices, out);
   serializeArray(*map->mapValues(), indices, out);