Remove unused variables in faiss/utils/hamming_distance/neon-inl.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.

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Reviewed By: palmje

Differential Revision: D53779537

fbshipit-source-id: 698cdc6720c10d219e56ecd2aad8f4cbe636c639

Author: r-barnes

faiss/utils/hamming_distance/neon-inl.h

@@ -260,7 +260,6 @@ struct HammingComputer32 {
     }
 
     inline int hamming(const uint8_t* b8) const {
-        const uint64_t* b = (uint64_t*)b8;
         uint8x16_t b0 = vld1q_u8(b8);
         uint8x16_t b1 = vld1q_u8(b8 + 16);