[Figma] remove placeholder text from dropdown and it's variants

[laurenmrice]

After discussions with Accessibility we have decided to remove current placeholder text from the fields of Dropdown, Combobox Multiselect, and Filterable Multiselect.

However, the Multiselect and Filterable Multiselect selected states can still provide text in the field along with the number tag to help inform the user that options(s) are selected.

• Start this issue once it has been implemented in code #14308
  • Update from Dev: Updating fields in Dropdown to have no placeholder text is considered a breaking change and can not be implemented right now in code.

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Design spec

[laurenmrice]

• White theme
• Publish