Remove broken sibling selector optimization

Blink r154053 landed with a broken optimization for nth-child that
this patch removes.

Background: nth-child was formerly O(1) and nth-last-child was O(N^2),
but r154053 reversed these making the more common nth-child slower.
It included an optimization to make both O(1), but the optimization
was broken in one of the merges.

I will re-land the working optimization in another patch, and with
a magnitude perf test Ojan, eseidel, and myself wrote to ensure it

Review URL:

git-svn-id: svn:// bbb929c8-8fbe-4397-9dbb-9b2b20218538
8 files changed