lang: remove param label renaming syntax

[sbillig]

I declare that this feature is unnecessary. It adds mental and syntax overhead for little benefit. (I'm the one that decided we needed it in the first place, of course).

Arg labels are still required, unless preceded by an underscore.

// this is no longer allowed:
fn frob(knob x: u32) { // call site: "frob(knob: 10)", arg is called "x" internally
  if x == 0 { .. }
}

// arg labels are still required at the call site:
fn frob(knob: u32) { // call `frob(knob: 10)`
  // if you want to use the name `x` internally, just make an alias:
  let x = knob
}

// use underscore to not require arg labels
fn add(_ x: u32, _ y: u32) -> u32 { .. } // call `add(10, 20)`

[cburgdorf]

Fine by me