Remove feature gates for if let, while let, and tuple indexing

[nrc]

Closes #19469

r?

[alexcrichton]

@nick29581 could you actually outright delete the features instead? I still need to document this, but I don't think we want to deal with the Accepted state any more (you should just remove the name of the feature).

[nrc]

@alexcrichton should we leave them as Accepted for a while so that people get warned about unnecessary feature opt ins? (I guess otherwise they just get an unnecessary feature gate error, which is not as nice, but not too bad).

[sfackler]

They can't be deleted outright until after this lands in a snapshot in any case.

[alexcrichton]

@nick29581 possibly yes, but as #![feature] is a statement of intent that you're willing to accept breakage it's not strictly necessary that we have a warning period. @sfackler is right though in that I think these are required, so don't mind me!

[steveklabnik]

Even then, they can't be deleted. From a few lines above:

https://github.com/nick29581/rust/blob/iflet/src/libsyntax/feature_gate.rs#L35-L37

[alexcrichton]

Yeah I wrote that comment when I initially implemented feature gates, but they have changed quite significantly since then and the comment is out of date at this point.