Remove unused constant: `Vulnerability::TooManyDaysError`

The constant TooManyDaysError in the vulnerability model appears to be unused. From some git spelunking:

1. This was added in commit 5825c12f
2. It was also used in 7ff151eb
3. The resolver from 2 removed in 89009fc7
4. The usage from 1 was removed in e9567195