[Screen reader] Remove aria for field_dropdown image element

[microbit-robert]

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Description

In #9384, role and aria-label were added to the image element (if applicable) for field_dropdown. This causes the dropdown to be read as 'label, image / graphic' rather than providing list-box information and using the aria-label of the parent g element. I think these attributes should probably be removed so the more informative aria information is used.

Reproduction steps

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Stack trace

Screenshots

MakeCode examples with VoiceOver below.

If the image has a role and aria-label:

If the image does not have a role and aria-label:

Browsers

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