Backdrop - Remove "With Spinner" example

[mcarrano]

Want to remove the example of a Backdrop with a spinner (https://www.patternfly.org/v4/components/backdrop#with-spinner) as this is something we don't intend to support.

See discussion here: patternfly/patternfly#5248

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[mcarrano]

@tlabaj @nicolethoen can we just remove this example from the React page?