fix(mixins): border none is missing from new button mixin

[joestr]

Proposed changes

A text button with either btn text or btn-flat has a border. During the move to mixins a border: none got lost.

Screenshots (if appropriate) or codepen:

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).

Checklist:

• I have read the CONTRIBUTING document.
• My commit messages follows the conventional commit format
• My change requires a change to the documentation, and updated it accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[wuda-io]

LGTM 👍

[wuda-io]

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