Remove unused assert_redirected_to_placeholder_page method
Remove unused assert_redirected_to_placeholder_page method

This was made redundant in 616744be72622851a3d0ae476de29ba9bb22ef48 (Sep
2013).

I noticed it when fixing some tests in this file in the previous commit.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall