Timeout on iOS after clearWatch

iOS 11.4.1

I experience a timeout error after calling `watchPosition `and then calling `clearWatch `in the first success callback.

Example code would be some thing like
```
var count = 0
var watchID = navigator.geolocation.watchPosition(onSuccess, onError, { enableHighAccuracy: true });

var onSuccess= function (position) {
if (count === 0) {
navigator.geolocation.clearWatch(watchID);
}
count ++;
}

function onError(error) {
console.log('code: ' + error.code + '\n' +
'message: ' + error.message + '\n');
}
```
What I believe is happening is
1. `watchPosition `calls `geolocation.getCurrentPosition` followed by
1. `exec(win, fail, 'Geolocation', 'addWatch', [id, options.enableHighAccuracy]);`
1. The `onSuccess `callback following step 1 calls `clearWatch`
1. The `win `callback following step 2 restarts the timeout
1. The timeout is hit because the geolocation was stopped in step 3

This was unexpected behaviour which I only figured out by looking at the source code of the plugin. I could easily resolve the problem by calling clearWatch a second time but it would be nicer for the plugin to handle this.