Drop unused variables in utility.cc (#7494)
Drop unused variables in utility.cc (#7494)

The HMAC() and EVP_DigestFinal() functions accept nullptr
insted of an int* when the caller doesn't care about the
length.

Signed-off-by: Raul Gutierrez Segales <f25f6873bbbde69f1fe653b3e6bd40d543b8d0e0@pinterest.com>

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