bench: remove unnecessary & incorrect multiplication in MuHashDiv

Introduced in #19055, MuHashDiv benchmark used to multiply with a loop
based on epochIterations. That does not do what it is supposed to do,
because epochIterations() is determined automatically from nanobench.

Also, multiplication is not needed for the algorithm (as pointed out by
a comment in #19055), so it's better to remove this loop.

src/bench/crypto_hash.cpp

@@ -134,10 +134,6 @@ static void MuHashDiv(benchmark::Bench& bench)
     FastRandomContext rng(true);
     MuHash3072 muhash{rng.randbytes(32)};
 
-    for (size_t i = 0; i < bench.epochIterations(); ++i) {
-        acc *= muhash;
-    }
-
     bench.run([&] {
         acc /= muhash;
     });