Remove border of disabled calendar buttons in Dropdowns example

Description

Add a new rule in examples/dropdowns/dropdowns.css to remove the border for disabled calendar buttons.

Motivation & Context

This change is required to retrieve the same rendering before ef9d8538 that introduces for the buttons:

&:disabled,
&.disabled,
fieldset:disabled & {
  /*...*/
  border-color: var(--#{$variable-prefix}btn-disabled-border-color);
  /*...*/
}

Rendering of the current main branch to compare to the current Dropdowns example:

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

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