Remove custom divider styles in icons.css

[wise-king-sullyman]

Blocked by https://github.com/patternfly/patternfly-react/issues/10390

When/if the above enhancement is complete we should see if we can use that new functionality to get rid of the custom styles we have for a divider in the icons stylesheet.

Followup from https://github.com/patternfly/patternfly-org/pull/4011#discussion_r1605141970

CC @mcoker

[nicolethoen]

Do this as a follow up to https://github.com/patternfly/patternfly-org/pull/4102

[nicolethoen]

Closed by #4146