Remove unused `MAX_DAYS_OF_HISTORY` from vulnerabilities

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
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git history

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