Remove unused variables in faiss/utils/hamming.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: danzimm

Differential Revision: D52847958

fbshipit-source-id: b3f98d8e0e4fd0e13191f0c0d907edd7c0e78db0

Author: r-barnes

faiss/utils/hamming.cpp

@@ -35,8 +35,6 @@
 #include <faiss/utils/approx_topk_hamming/approx_topk_hamming.h>
 #include <faiss/utils/utils.h>
 
-static const size_t BLOCKSIZE_QUERY = 8192;
-
 namespace faiss {
 
 size_t hamming_batch_size = 65536;