docs: remove outdated blank screen issue

[MikeMcC399]

Issue

README > Common problems > Blank screen in Chrome contains the following advice:

When running headed tests with X11 forwarding in Cypress v4 you might see a blank Chrome screen. Try disabling memory sharing by setting the following environment variables:

export QT_X11_NO_MITSHM=1
export _X11_NO_MITSHM=1
export _MITSHM=0

• See issue #270

• issue #270 was closed in March 2020.

---

• The variables mentioned have progressed through to
  

  cypress-docker-images/factory/factory.Dockerfile

  Lines 17 to 21 in 47149ea

  	# disable shared memory X11 affecting Cypress v4 and Chrome
  	# https://github.com/cypress-io/cypress-docker-images/issues/270
  	QT_X11_NO_MITSHM=1 \
  	_X11_NO_MITSHM=1 \
  	_MITSHM=0 \

  where these are promulgated to all current Cypress Docker images.

Change

Since a manual workaround is no longer necessary, the section README > Common problems > Blank screen in Chrome is removed from the README document.

[cypress-app-bot]

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