Remove redundant constraints on existentials/mirrors

Author: airspeedswift

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -427,8 +427,8 @@ internal class _AnyRandomAccessCollectionBox<Element>
 @_versioned
 internal final class _${Kind}Box<S : ${Kind}> : _Any${Kind}Box<S.Element>
   where
-  S.SubSequence : ${Kind},
 %  if Kind == 'Sequence':
+  S.SubSequence : ${Kind},
   S.SubSequence.Element == S.Element,
   S.SubSequence.SubSequence == S.SubSequence
 %  else:
@@ -1040,7 +1040,6 @@ public struct ${Self}<Element>
     where
     // FIXME(ABI)#101 (Associated Types with where clauses): these constraints should be applied to
     // associated types of Collection.
-    C.SubSequence : ${SubProtocol},
     C.SubSequence.Element == Element,
     C.SubSequence.Indices : ${SubProtocol},
     C.Indices : ${SubProtocol}

stdlib/public/core/Mirror.swift

@@ -218,7 +218,6 @@ public struct Mirror {
     C.Element == Child,
     // FIXME(ABI)#47 (Associated Types with where clauses): these constraints should be applied to
     // associated types of Collection.
-    C.SubSequence : Collection,
     C.SubSequence.Indices : Collection,
     C.Indices : Collection {
 
@@ -270,7 +269,6 @@ public struct Mirror {
   ) where
     // FIXME(ABI)#48 (Associated Types with where clauses): these constraints should be applied to
     // associated types of Collection.
-    C.SubSequence : Collection,
     C.Indices : Collection {
 
     self.subjectType = Subject.self