Tests: Remove redundant ThemeProvider from tests

[siddharthkp]

There are multiple tests that wrap the rendered component with ThemeProvider: https://github.com/search?q=repo%3Aprimer%2Freact+%3CThemeProvider+path%3Atest.tsx&type=code

We do not have to do this anymore because we already wrap all stories in a ThemeProvider in preview.js.

Note: There might be a couple exceptions so make sure to test this by switching themes in storybook