Nearing retirement - thresholds changed

https://www.pivotaltracker.com/story/show/59817116
"Currently functions when someone is within 4 months and 1 day from
state pension age but it should be within 4 months and up to 1 day
before state pension age."
Remove obsolete within_four_months test

Logic has changed so the first of these two no longer passes.
Edge cases are fully tested in new unit tests so removed both.