decide on final context function syntax

[bishabosha]

Minimized code

The following are both valid, I believe we should be removing the form with using clause based on #10487

val f = (using x: Int) => x
val g = (x: Int) ?=> x

Expectation

Only val g = (x: Int) ?=> x should work

[b-studios]

For what it's worth, #10484 is the issue containing the discussion of switching back to the operator style.