Remove ability to specify multiple training dataloaders

[VladVin]

Is your feature request related to a problem? Please describe.
It is impossible to use multiple training datasets according to Lightning's interface (there is no dataloader_idx parameter for training_step).
But TorchOk supports a list of training datasets though it won't work - the user has to specify only one training dataloader to make it work.

Describe the solution you'd like
Remove the ability to pass multiple training dataloaders

Describe alternatives you've considered
We can also create our own wrapper around a list of datasets, but it will be confusing for a user

Additional context