refactor(@angular/cli): remove outdated 1.x karma plugin error placeholder

For Angular 6, an error inducing file was introduced in the 1.x location of the karma plugin for the Angular CLI.  This was useful during the 1.x -> 6 transition to assist users to migrate with a helpful error message.  Now that 1.x and 6 are no longer supported, this can be removed.

Author: clydin