Tidy up KernelArgumentBlock::getauxval.

Correct the comment, and remove the unused functionality. getauxval(3) does
now set errno to let you know it failed to find anything, but since none of
this function's callers care anyway it seems safer to leave errno untouched
until we actually have a demonstrated need for it.

Bug: https://code.google.com/p/android/issues/detail?id=198111
Change-Id: I232a42dc5a02c8faab94c7d69bef610408276c23