Thanks, nice improvement! code lgtm!

Thx for all those changes! ❤️ So I did some 🥣 a-la-carte testing

1. I've installed our ecommerce sample data kibana_sample_data_ecommerce (timeField order_date)
2. I created a new data view *ecommerce* (timeField: products.created_on)
3. I've selected kibana_sample_data_ecommerce in Discover, no changes
4. When I switched from kibana_sample_data_ecommerce (no sort change) to *ecommerce* , the documents were now sorted by products.created_on ... this is how it should behave 👍

Before it was still sorted by order_date

1. Now I switched to kibana_sample_data_ecommerce,
2. Sorted by products.product_name.keyword,order_date, so it's a non default sort with the timefield on the second place.
3. Switched to *ecommerce*
4. The result was, that the sorting was kept, which I expected to be fine, but since there's a different time column configured, there's no more way to remove the sorting unless changing the URL:
   
   So I think we need to consider also this case, but I'm not 100% sure how.
   There are 2 ways
5. We remove the timefield of the previous data field in this case (🚀)
6. We replace it with the timefield the user has just selected (❤️ )
   Adding another 2 pair of eyes here @majagrubic @timroes

added 2 hopefully last comments. thx so much 🙏 for your patience

LGTM, thx so much! 👍
Final test by using this data (one of the examples of the issues linked)

Steps to reproduce:
Create 2 indices, and 2 index patterns: timefield is the time field of my_index and timefield_2 is the time field of my_index_2. In my_index_2 also add timefield to the mapping but it won't contain data:

PUT my_index
{
  "mappings": {
    "properties": {
      "timefield": {
        "type": "date"
      }
    }
  }
}

POST my_index/_bulk
{ "index": {} }
{ "timefield": "2021-08-05" }
{ "index": {} }
{ "timefield": "2021-08-04" }
{ "index": {} }
{ "timefield": "2021-08-03" }

PUT my_index_2
{
  "mappings": {
    "properties": {
      "timefield": {
        "type": "date"
      },
      "timefield_2": {
        "type": "date"
      }
    }
  }
}

POST my_index_2/_bulk
{ "index": {} }
{ "timefield_2": "2021-08-05" }
{ "index": {} }
{ "timefield_2": "2021-08-04" }
{ "index": {} }
{ "timefield_2": "2021-08-03" }

Now in Discover, when switching from my_index to my_index_2, Discover sees the current sorting field (timefield) exists in my_index_2 and doesn’t change it. Since there is no data in this field in my_index_2, it shows up as unsorted.

With this PR
It's fixed 🎂 , switching the to my_index_2 also switches the timeField, so it works