docs: remove placeholder text from dropdown images and content

[laurenmrice]

Ref #3526

Based off of a11y considerations and for content consistency, we need to remove placeholder text from Dropdown, Multiselect, and Combobox.

• Remove from Usage tab content and examples
• Remove from Style tab content and examples

*Note: This work is blocked an can not be started until it is in code. This is considered a breaking change and will have to be implemented at a later date.

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