[Stdlib] Remove ABI FIXMEs swiftlang#94 and swiftlang#96, which are consequences of other FIXMEs.

There isn't any work related to SE-0142 associated type where clauses
for these particular ABI FIXMEs. Rather, we get all of the constraints
we need from Collection and the recursive constraint we cannot yet
express, all of which are covered by other ABI FIXMEs.

Author: DougGregor

stdlib/public/core/BidirectionalCollection.swift

@@ -87,19 +87,13 @@ public protocol BidirectionalCollection
   associatedtype SubSequence : _BidirectionalIndexable, Collection
     = BidirectionalSlice<Self>
   // FIXME(ABI)#93 (Recursive Protocol Constraints):
-  // FIXME(ABI)#94 (Associated Types with where clauses):
-  // This is dependent on both recursive protocol constraints AND associated 
-  // types with where clauses.
   // associatedtype SubSequence : BidirectionalCollection
 
   /// A type that represents the indices that are valid for subscripting the
   /// collection, in ascending order.
   associatedtype Indices : _BidirectionalIndexable, Collection
     = DefaultBidirectionalIndices<Self>
   // FIXME(ABI)#95 (Recursive Protocol Constraints):
-  // FIXME(ABI)#96 (Associated Types with where clauses):
-  // This is dependent on both recursive protocol constraints AND associated 
-  // types with where clauses.
   // associatedtype Indices : BidirectionalCollection
 
   /// The indices that are valid for subscripting the collection, in ascending