remove zIndex property

[dzonidoo]

SDESK-7429

Front-end checklist

• This pull request is adding missing TypeScript types to modified code segments where it's easy to do so with confidence
• This pull request is using TypeScript interfaces instead of prop-types and updates usages where it's quick to do so
• This pull request is using memo or PureComponent to define new React components (and updates existing usages in modified code segments)
• This pull request is replacing lodash.get with optional chaining and nullish coalescing for modified code segments
• This pull request is not importing anything from client-core directly (use superdeskApi)
• This pull request is importing UI components from superdesk-ui-framework and superdeskApi when possible instead of using ones defined in this repository.
• This pull request is not using planningApi where it is possible to use superdeskApi
• This pull request is not adding redux based modals
• In this pull request, properties of redux state are not being passed as props to components; instead, we connect it to the component that needs them. Except components where using a react key is required - do not connect those due to performance reasons.
• This pull request is not adding redux actions that do not modify state (e.g. only calling angular services; those should be moved to planningApi)