DOCS-895 - Remove XHR metrics from RUM docs

[kimsauce]

Purpose of this pull request

Remove XHR metrics from RUM docs

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

https://sumologic.atlassian.net/browse/DOCS-895

[jpipkin1]

Approved, @kimsauce. But it looks like there might be a fix for the issue that may not require updating documentation. See Slack.

[kimsauce]

Approved, @kimsauce. But it looks like there might be a fix for the issue that may not require updating documentation. See Slack.

Thank you, @jpipkin1 !