Prior to this change, `TargetPane` passed `isRtl` to
`TargetPaneComponent` which then passed it into a `div`, causing the
warning below. Since `TargetPaneComponent` doesn't currently use `isRtl`
so there's no need to pass it in. If `TargetPaneComponent` starts
needing `isRtl` then we should adjust it to not pass that property to
any DOM elements.

The console warning in question is 57 lines long and starts with:

Warning: React does not recognize the `isRtl` prop on a DOM element. If
you intentionally want it to appear in the DOM as a custom attribute,
spell it as lowercase `isrtl` instead. If you accidentally passed it
from a parent component, remove it from the DOM element.