base: Drop out-of-date comment
The comment in GHC.Base claimed that ($) couldn't be used in that module as it was wired-in. However, this is no longer true; ($) is merely known key and is defined in Haskell (with a RuntimeRep-polymorphic type) in GHC.Base.
The one piece of magic that ($) retains is that it a special typing
rule to allow type inference with higher-rank types
(e.g. runST $ blah
; see Note [Typing rule for ($)] in TcExpr).