Metrics: Delete UI for Monitor Metrics view

⚠ Removal should take place in %16.0 or later.

As part of #346541 (closed) remove Monitor > Metrics view and left navigation item

Relevant files

frontend

• app/assets/javascripts/monitoring/*
• app/views/projects/environments/metrics.html.haml
• app/assets/stylesheets/page_bundles/prometheus.scss
• spec/frontend/monitoring
• lib/sidebars/projects/menus/monitor_menu.rb

Implementation plan

1. delete Monitor > Metrics view and left navigation item

Verification steps

Double check the Monitor menu does not 404 without the left nav item, per #280564 (comment 857579682).

These areas of the app should remain unavailable & without regressions to other features: