Remove unused `MAX_DAYS_OF_HISTORY` from vulnerabilities

What does this MR do and why?

The MAX_DAYS_OF_HISTORY constant in the vulnerability model appears to be unused:

~/gitlab-development-kit/gitlab$ ag --ignore TAGS \
                                 --literal \
                                 --group \
                                 --line-number \
                                 --column \
                                 --smart-case -- MAX_DAYS_OF_HISTORY .
File: ee/app/models/ee/vulnerability.rb
22:7:      MAX_DAYS_OF_HISTORY = 10
1 matches
1 files contained matches
62376 files searched

git history

1. added in 7ff151eb
   • used in VulnerabilitiesHistoryResolver and Vulnerability.counts_by_day_and_severity
2. VulnerabilitiesHistoryResolver removed in 89009fc7
3. Vulnerability.counts_by_day_and_severity removed in e9567195

resolves: #433494 (closed)

How to set up and validate locally

1. ci passes
2. run a similar search as the ag above

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #433494 (closed)