Get rid of TreeScope::noDocumentInstance()

The function was introduced to handle Node instances which don't have
documents, that did happen only with DocumentType class. It is no
longer possible for DocumentType to have null Document after r156581.

This change eliminates the complexity to follow the simplification
at r156581.

R=adamk,esprehn
BUG=none
TEST=none

Review URL: https://codereview.chromium.org/99103009

git-svn-id: svn://svn.chromium.org/blink/trunk@163747 bbb929c8-8fbe-4397-9dbb-9b2b20218538