fix(material/select): don't announce selected value multiple times

[mhaertwig]

The screen reader already announces the selected value inside the select field. Therefore, pointing the aria-labelledby attribute to the value field made the screen reader announce the selected value multiple times. This pull request fixes this by removing the value field from aria-labelledby.

Before this fix:

After this fix:

[crisbeto]

What screen readers did you test this with? If I remember correctly, we had to do this because a specific screen reader wasn't reading the value out automatically.

[mhaertwig]

I've tested with NVDA, JAWS and Narrator. It works well with NVDA and JAWS. Narrator still sometimes reads the selected value twice but before it read it three times without any break.

[mhaertwig]

@crisbeto, you were right: The Chrome plugin "Screen Reader" did not read the value of the combobox after the aria-labelledby pointing to the value was removed. I've investigated this issue further and finally found a quite simple solution: #24831

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