The `TokenBalance` component now uses the new `useTokenTracker` hook
instead of the `withTokenTracker` higher-order component. The component
was converted from a split container/component to a functional
component so that the hook could be used.
The prop `withSymbol` was removed from a few call sites, as it was no
longer used. An empty string was substituted for any falsy `string` or
`symbol` because that's what the `withTokenTracker` higher-order
component did.
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