Remove multiple client support from HTMLImportChild

This reverts r164060 because it turned out that it never has
two or more clients.

TEST=none
BUG=none
R=haraken@chromium.org, haraken, tkent

Review URL: https://codereview.chromium.org/144873002

git-svn-id: svn://svn.chromium.org/blink/trunk@165575 bbb929c8-8fbe-4397-9dbb-9b2b20218538