Proposal: Remove language.postfixOps entirely in Scala 3.10

[SethTisue]

this is the next logical step after #5498

the deprecate-and-remove process began all the way back in Scala 2.10 (2012) when we started warning:

Welcome to Scala version 2.10.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_80).

scala> Nil size
warning: there were 1 feature warning(s); re-run with -feature for details
res0: Int = 0

I suggest that 3.10 is high time to finish the removal.