Remove dead code from childIndex optimization

We used to use the childIndex in the DOMSiblingTraversalStrategy but
leviw@ removed it to fix a bug after he reversed the direction we did
recalcStyle in r160288 and r158839. We're going to need something like
this eventually, but I think we want something even better than this
old optimization since it didn't help querySelector at all.

Lets just delete this code for now while we figure out the right way
to optimize nth-child selectors.

BUG=364081

Review URL: https://codereview.chromium.org/245103004

git-svn-id: svn://svn.chromium.org/blink/trunk@172105 bbb929c8-8fbe-4397-9dbb-9b2b20218538