No need for EphemeronTop since ephemerons are covariant.

Closes PR 11633.

original commit: 9740d5542b1cd0046ebdbb1c28ba73ea66025366

collects/typed-scheme/private/base-env.rkt

@@ -25,7 +25,6 @@
  (only-in (types abbrev numeric-tower) [-Number N] [-Boolean B] [-Symbol Sym])
  (only-in (rep type-rep) make-HashtableTop make-MPairTop
           make-BoxTop make-ChannelTop make-VectorTop
-          make-EphemeronTop
           make-Ephemeron
           make-HeterogenousVector))
 
@@ -1034,6 +1033,6 @@
 
 ;ephemerons
 [make-ephemeron (-poly (k v) (-> k v (make-Ephemeron v)))]
-[ephemeron? (make-pred-ty (make-EphemeronTop))]
+[ephemeron? (make-pred-ty (make-Ephemeron Univ))]
 [ephemeron-value (-poly (v) (-> (make-Ephemeron v) (Un (-val #f) v)))]
 

collects/typed-scheme/rep/type-rep.rkt

@@ -306,7 +306,6 @@
 ;; the supertype of all of these values
 (dt BoxTop () [#:fold-rhs #:base] [#:key 'box])
 (dt ChannelTop () [#:fold-rhs #:base] [#:key 'channel])
-(dt EphemeronTop () [#:fold-rhs #:base] [#:key 'ephemeron])
 (dt VectorTop () [#:fold-rhs #:base] [#:key 'vector])
 (dt HashtableTop () [#:fold-rhs #:base] [#:key 'hash])
 (dt MPairTop () [#:fold-rhs #:base] [#:key 'mpair])

collects/typed-scheme/types/printer.rkt

@@ -125,7 +125,6 @@
     [(StructTop: st) (fp "~a" st)]
     [(BoxTop:) (fp "Box")]
     [(ChannelTop:) (fp "Channel")]
-    [(EphemeronTop:) (fp "Ephemeron")]
     [(VectorTop:) (fp "Vector")]
     [(MPairTop:) (fp "MPair")]
     [(App: rator rands stx) 

collects/typed-scheme/types/subtype.rkt

@@ -360,7 +360,6 @@
               ;ephemerons are covariant
               [((Ephemeron: s) (Ephemeron: t))
                (subtype* A0 s t)]
-              [((Ephemeron: _) (EphemeronTop:)) A0]
               [((Box: _) (BoxTop:)) A0]
               [((Channel: _) (ChannelTop:)) A0]
               [((Vector: _) (VectorTop:)) A0]