Do not use MD5SumValue for Description_md5()

Our profile says we spend about 5% of the time transforming the
hex digits into the binary format used by HashsumValue, all for
comparing them against the other strings. That makes no sense
at all.

According to callgrind, this reduces the overall instruction
count from 5,3 billion to 5 billion in my example, which
roughly matches the 5%.