Remove implementation of Entity constructor

Remove implementation of Entity constructor as we never construct Entity
objects. This will lead in a link time error if someone tries to use it
instead of a runtime error (due to the ASSERT_NOT_REACHED() in the
constructor).

This gets rid of a case where 0 is passed as TreeScope to the ContainerNode /
Node constructor.

R=adamk

Review URL: https://codereview.chromium.org/62843007

git-svn-id: svn://svn.chromium.org/blink/trunk@162041 bbb929c8-8fbe-4397-9dbb-9b2b20218538