Get rid of hazardous EVP_DigestInit_dbg/EVP_DigestInit case

distinction (which does not work well because if CRYPTO_MDEBUG is
defined at library compile time, it is not necessarily defined at
application compile time; and memory debugging now can be reconfigured
at run-time anyway).  To get the intended semantics, we could just use
the EVP_DigestInit_dbg unconditionally (which uses the caller's
__FILE__ and __LINE__ for memory leak debugging), but this would make
memory debugging inconsistent.  Instead, callers can use
CRYPTO_push_info() to track down memory leaks.