Remove Capybara-based rspec integration

Why is this change being made?

As we have disabled Capybara-based integration test with RSpec in our CI since 2020-03-25 (UTC) (almost 1 year) (cf. !44516 (merged)), now we can remove Capybara dependency as well as CI manifest (.gitlab-ci.yml) entirely.

This resolves a blocker (due to old version of Capybara) against !75753 (closed).

See thread here for more context on why we don't think we need integration tests at this point.

Tasks

• Remove Capybara and all related dependencies
  • selenium-webdriver should be able to be deleted too.
• Remove references in ci config and build
• Remove spec/features, as well as any files/methods which are only used to support those specs.
• All references to feature or features should be gone from spec/spec_helper
  • The entire before(:suite) should be able to be deleted.
• Remove related dependences from Docker image (e.g. browser-related dependencies, chromedriver, etc.) - will need to be separate MR in docker image repo, link here for reference. gitlab-org/gitlab-build-images#70 (closed)
• Remove references in doc/development.md and any other references in docs.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)

NOTE: These can be re-enabled once the handbook is moved to its own separate middleman instance and pipeline, which hopefully will eliminate the middleman flakiness. See the Handbook Monorepo Refactor Epic for more details

Replaces !75779 (closed)