Remove unused usage_ping partial

app/views/instance_statistics/cohorts/_usage_ping.html.haml

-%h2#usage-ping Usage ping
-
-.bs-callout.clearfix
-  %p
-    User cohorts are shown because the usage ping is enabled. The data sent with
-    this is shown below. To disable this, visit
-    = succeed '.' do
-      = link_to 'application settings', admin_application_settings_path(anchor: 'usage-statistics')
-
-%pre.usage-data.js-syntax-highlight.code.highlight{ data: { endpoint: usage_data_admin_application_settings_path(format: :html) } }