Remove deprecated extension method syntax

[eloots]

Minimized example

In 0.27.0-RC1 and 0.28.0 nightly build, the "old" syntax for defining extension methods is still supported, e.g.:

def (c: Circle) circumference: Double = c.radius * math.Pi * 2

This should no longer compile [at least at some point in time before Scala 3 is released].

Expectation

Even though there are arguments for keeping this old syntax supported to allow people to transition to the new syntax, it would be a good idea to not delay removing support for it.