[Transform] Allow transforms to use PIT with remote clusters again

[droberts195]

In #99803 we disabled the use of PIT for transforms with remote source indices. This was done because, at that time, PIT could be very inefficient and unreliable in the presence of many thousands of source indices, and CCS scenarios are most likely to have thousands of source indices.

Now that #104288 has been fixed it should be safe to revert the changes of #99803. This isn't desperately urgent but should not be forgotten. We should aim to do it for 8.13 if there's time.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)