Dont repeat voices in speechsynthesis demo

[sjdallst]

What this PR does

Makes it so the select element in the speechsynthesis demo is cleared every time loadVoices is called. Previously the code was not clearing the box, which caused the list to repeat voices. @molant

Requirements

• [NA] My PR follows all applicable accessibility requirements (See .github/ACCESSIBILITY_REQS.md).
• [NA] My PR follows the CSS code style guidelines (See .github/CSS_STYLE_REQS.md).
• [Y] I have linted my code using npm run lint:css -- demoDirectoryName/**/*.css and npm run lint:js -- demoDirectoryName/**/*.js, and have fixed the errors.

[msftclas]

All CLA requirements met.

[molant]

Thanks @sjdallst !