API: Use delete instead of remove in actions for clarity

[aokolnychyi]

This PR switches our new actions to use delete instead of remove as I think delete makes more sense given the naming of methods in these actions.

This is NOT a breaking change as we modify only the unreleased Actions API.

[aokolnychyi]

Now it matches the naming in ExpireSnapshots.
The downside is that we will add deletedDeleteFilesCount in the future. I can live with that.

[aokolnychyi]

@RussellSpitzer @flyrain @karuppayya @rdblue, I took a look at the current state of our new actions and I think it would make sense to switch from RemoveXXX to DeleteXXX in names.

Let me know what you think.

[RussellSpitzer]

My only worry here is we have two senses of delete in the API. Logical deletes and physical deletes, before the names were distinct since "delete" apis only did logical deletes while "remove" apis actually removed files from your file system. I also only worry about this on about a 2 out of 10 worry scale so i'm +1 on moving forward if @aokolnychyi feels strongly about it :)

[rdblue]

I'm okay with either remove or delete.

[aokolnychyi]

My primary motivation is the inconsistency we have in the code right now.

In three actions (including RemoveOrphanFiles), we have deleteWith and executeDeleteWith methods that configure how to do the actual removal. We also use deletedXXXCounts in ExpireSnapshots to report the number of potentially physically removed files.

Karuppayya's original implementation called the action DeleteReachableFiles and I asked to change that to use remove but I've changed my opinion after looking at the code a little bit more.