Remove obsolete (and duplicate) error alert

[kulmann]

Description

Somehow the old oc-alert snuck in in the dialog prompt. When squashing the recent PR which added the new error message in oc-text-input I dropped a commit - I suppose that removing the old oc-alert was part of that commit and I've probably missed that... This PR fixes the dialog prompt by removing it again.

Motivation and Context

Dialog prompt shows the error message twice right now. Once within the oc-text-input component and once as an alert below. Obviously the alert has to be removed.

How Has This Been Tested?

Locally in Chrome and Firefox.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.