HTML Imports: Get rid of outdated "removed-flag" concept.

As it is remove from the spec, the implementation follow.

TEST=import-element-removed-flag.html,import-remove.html
BUG=343706
R=dglazov@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/198103005

git-svn-id: svn://svn.chromium.org/blink/trunk@169155 bbb929c8-8fbe-4397-9dbb-9b2b20218538