MINOR: tools: don't use unlikely() in hex2i()

MINOR: tools: don't use unlikely() in hex2i()

This small inline function causes some pain to the compiler when used
inside other functions due to its use of the unlikely() hint for non-digits.
It causes the letters to be processed far away in the calling function and
makes the code less efficient. Removing these unlikely() hints has increased
the chunk size parsing by around 5%.