Remove security dashboard store

What does this MR do and why?

This is part of the frontend cleanup after the pipeline_security_dashboard_graphql feature flag removal.

There will be more MRs to follow as the total cleanup is very big. This MR follows

1. Remove security_dashboard from pipeline_securit... (!167903 - merged)
2. Remove unused security_dashboard_vuex and loadi... (!168496 - merged)
3. Remove pipeline filters and spec (!168498 - merged)
4. Remove unused vuln pipeline components (!168499 - merged)

Check #438235 (closed) for a list of the full cleanup.

This MR removes the security dashboard store with all its modules (their actions, mutations, state, getters, mutation types), mediator plugin and their spec files. It also moves mock_data_vulnerabilities.js from ee/spec/frontend/security_dashboard to ee/spec/frontend/dependencies because the mock is still used there.

Within the store path ee/app/assets/javascripts/security_dashboard/store/modules we still find vulnerabilities/constants.js which includes LOADING_VULNERABILITIES_ERROR_CODES. This constant is still used in ee/app/assets/javascripts/security_dashboard/utils/pipeline_report_options.js and will be part of a next cleanup MR.

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Related to #438235 (closed)