Remove dead code from LiveNodeListBase

Remove dead code from LiveNodeListBase. The m_isItemRefElementsCacheValid
member and its associated getter / setter are never used.

R=abarth, esprehn

Review URL: https://codereview.chromium.org/109013007

git-svn-id: svn://svn.chromium.org/blink/trunk@164322 bbb929c8-8fbe-4397-9dbb-9b2b20218538