Remove already-completed or separate-discussed ABI FIXMEs.

Author: DougGregor

stdlib/public/core/BidirectionalCollection.swift

@@ -66,8 +66,6 @@ public protocol _BidirectionalIndexable : _Indexable {
 /// - If `i > c.startIndex && i <= c.endIndex`
 ///   `c.index(after: c.index(before: i)) == i`.
 public protocol BidirectionalCollection : _BidirectionalIndexable, Collection 
-// FIXME(ABI) (Revert Where Clauses): Restore these 
-// where SubSequence: BidirectionalCollection, Indices: BidirectionalCollection
 {
 
 // TODO: swift-3-indexing-model - replaces functionality in BidirectionalIndex

stdlib/public/core/Collection.swift

@@ -619,8 +619,6 @@ public struct IndexingIterator<
 /// the number of contained elements, accessing its `count` property is an
 /// O(*n*) operation.
 public protocol Collection : _Indexable, Sequence
-// FIXME(ABI) (Revert Where Clauses): Restore this
-// where Indices: Collection,
 {
   /// A type that represents the number of steps between a pair of
   /// indices.
@@ -649,15 +647,6 @@ public protocol Collection : _Indexable, Sequence
   associatedtype SubSequence : Collection = Slice<Self>
     where SubSequence.Index == Index
 
-  // FIXME(ABI)#99 (Associated Types with where clauses):
-  //   where SubSequence.Indices == Indices,
-  //
-  // (<rdar://problem/20715009> Implement recursive protocol
-  // constraints)
-  //
-  // These constraints allow processing collections in generic code by
-  // repeatedly slicing them in a loop.
-
   /// Accesses the element at the specified position.
   ///
   /// The following example accesses an element of an array through its