Unused `isPending` in `RouterContext`

[vasilii-kovalev]

The isPending flag in RouterContext is not used anywhere. Whether the UI is pending or not is decided by comparing data using location and nextLocation. So it seems the flag can be safely removed. May also require updating the documentation/notes.

[kentcdodds]

You're right. We don't technically need it, but I'm going to leave it in because exposing an isPending boolean from a router makes plenty of sense. Thanks for bringing it up, though.