Location Services: don't reset desiredAccuracy on every update/error (#23209)

Summary:
See https://github.com/facebook/react-native/issues/7680.

On iOS the RCTLocationObserver delegate is overriding `desiredAccuracy` every time CLLocationManager calls `didUpdateLocations` or `didFailWithError`.  `desiredAccuracy` is reset to
`RCT_DEFAULT_LOCATION_ACCURACY` (100 meters) This effectively makes it impossible for a react-native app to use any location accuracy other than the default.

This commit simply removes the code which resets the desired accuracy, as there seems to be no rationale for doing so. The reset code was added as part of [a large general
change](https://github.com/facebook/react-native/commit/705a8e0144775ff695f26a1f6b41d0baa6bed683) so the original intention is unclear from the history. If somebody can explain it to me, I'm happy to rework this PR accordingly.

Changelog:
----------
[iOS] [Fixed] - Location Services accuracy constantly reset to default of 100 meters.
Pull Request resolved: https://github.com/facebook/react-native/pull/23209

Differential Revision: D13879497

Pulled By: cpojer

fbshipit-source-id: f3c6c9c5ef698b23b99c407fd764ac990d69bf8c