Remove area chart in favor of time series

What does this MR do?

Removes an unused component:

• app/assets/javascripts/monitoring/components/charts/area.vue
• spec/javascripts/monitoring/charts/area_spec.js

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #66393 (closed)