Remove unused function on HTMLImportsController.

The showSecurityErrorMessage wasn't called by anything. So I killed it.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/305643009

git-svn-id: svn://svn.chromium.org/blink/trunk@175226 bbb929c8-8fbe-4397-9dbb-9b2b20218538