Merge branch 'productivity-analytics-docs-revert' into 'master'

Revert adding productivity analytics docs

See merge request gitlab-org/gitlab-ce!31848

doc/user/analytics/productivity_analytics.md

-# Productivity Analytics **(PREMIUM)**
-
-> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/12079) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.2 (enabled by feature flags `analytics` and `productivity_analytics`).
-
-Track development velocity with Productivity Analytics.
-
-For many companies, the development cycle is a blackbox and getting an estimate of how
-long, on average, it takes to deliver features is an enormous endeavor.
-
-While [Cycle Analytics](../project/cycle_analytics.md) focuses on the entire
-SDLC process, Productivity Analytics provides a way for Engineering Management to
-drill down in a systematic way to uncover patterns and causes for success or failure at
-an individual, project or group level.
-
-Productivity can slow down for many reasons ranging from degrading code base to quickly
-growing teams. In order to investigate, department or team leaders can start by visualizing the time
-it takes for merge requests to be merged.
-
-## Supported features
-
-Productivity Analytics allows GitLab users to:
-
-- Visualize typical Merge Request lifetime and statistics. Use a histogram
-  that shows the distribution of the time elapsed between creating and merging
-  Merge Requests.
-- Drill down into the most time consuming Merge Requests, select a number of outliers,
-  and filter down all subsequent charts to investigate potential causes.
-- Filter by group, project, author, label, milestone, or a specific date range.
-  Filter down, for example, to the Merge Requests of a specific author in a group
-  or project during a milestone or specific date range.
-
-## Accessing metrics and visualizations
-
-To access the **Productivity Analytics** page, go to **Analytics > Productivity Analytics**.
-
-The following metrics and visualizations are available on a project or group level:
-
-- Histogram showing the number of Merge Request that took a specified number of days to
-  merge after creation. Select a specific column to filter down subsequent charts.
-- Histogram showing a breakdown of the time taken (in hours) to merge a Merge Request.
-  The following intervals are available:
-  - Time from first commit to first comment.
-  - Time from first comment until last commit.
-  - Time from last commit to merge.
-- Histogram showing the size or complexity of a Merge Request, using the following:
-  - Number of commits per Merge Request.
-  - Number of lines of code per commit.
-  - Number of files touched.
-- Table showing list of Merge Requests with their respective times and size metrics.
-  Can be sorted by the above metrics.
-  - Users can sort by any of the above metrics
-
-## Retrieving data
-
-Users can retrieve three months of data when they deploy Productivity Analytics for the first time.
-
-To retrieve data for a different time span, run the following in the GitLab directory:
-
-```sh
-MERGED_AT_AFTER = <your_date> rake gitlab:productivity_analytics:recalc
-```
-
-## Permissions
-
-The **Productivity Analytics** dashboard can be accessed only:
-
-- On GitLab instances and namespaces on
-  [Premium or Silver tier](https://about.gitlab.com/pricing/) and above.
-- By users with [Reporter access](../permissions.md) and above.