[VSA] Remove deprecated methods

TODO after !59275 (merged) is deployed (next milestone)

• Remove the duration_chart_data method from ee/lib/ee/gitlab/analytics/cycle_analytics/data_collector.rb
• Remove the load method from ee/lib/gitlab/analytics/cycle_analytics/data_for_duration_chart.rb
• Remove the test cases as well.

The following discussion from !59275 (merged) should be addressed:

• @smcgivern started a discussion:

  Do we have an issue for this? Can we link to it here, please?