Remove the portions of these tests that were relying on old stdlib APIs.

Judging from history, the original tests appear to just be attempts to
flesh out test coverage rather than specific regression tests, and the
features in question are well-covered by other tests.

Author: rjmccall

test/Generics/associated_self_constraints.swift

@@ -54,19 +54,6 @@ class Subject<T>: Observer, Observable {
     }
 }
 
-/*
-FIXME: <rdar://problem/25666028> swift-3-indexing-model: Generics/associated_self_constraints.swift
-struct X<T> {
-  mutating func replace<
-    C : Collection where C.Iterator.Element == T, C.Index : Strideable
-  >(a: C) {
-    for i in a.startIndex..<a.endIndex {
-      _ = a[i] as T
-    }
-  }
-}
-*/
-
 protocol P {
     associatedtype A
 

test/Interpreter/protocol_extensions.swift

@@ -29,36 +29,6 @@ extension Collection {
 // CHECK: 4
 print(["a", "b", "c", "d"].clone().myCount)
 
-/*
-FIXME: <rdar://problem/25665969> swift-3-indexing-model: Interpreter/protocol_extensions.swift fails
-extension Collection {
-  final func indexMatching(_ fn: Iterator.Element -> Bool) -> Index? {
-    for i in myIndices {
-      if fn(self[i]) { return i }
-    }
-    return nil
-  }
-}
-
-// CHECK-FIXME: 2
-print(["a", "b", "c", "d"].indexMatching({$0 == "c"})!)
-
-// Extend certain instances of a collection (those that have equatable
-// element types) with another algorithm.
-extension Collection where Self.Iterator.Element : Equatable {
-  final func myIndexOf(_ element: Iterator.Element) -> Index? {
-    for i in self.indices {
-      if self[i] == element { return i }
-    }
-
-    return nil
-  }
-}
-
-// CHECK-FIXME: 3
-print(["a", "b", "c", "d", "e"].myIndexOf("d")!)
-*/
-
 extension Sequence {
   final public func myEnumerated() -> EnumeratedSequence<Self> {
     return self.enumerated()
@@ -110,19 +80,6 @@ extension MutableCollection
   }
 }
 
-/*
-FIXME: <rdar://problem/25665969> swift-3-indexing-model: Interpreter/protocol_extensions.swift fails
-// CHECK-FIXME: 4 3 1 2 | 5 9 8 6 7 6
-var evenOdd = [5, 3, 6, 2, 4, 9, 8, 1, 7, 6]
-var evenOddSplit = evenOdd.myPartition()
-for i in evenOdd.myIndices {
-  if i == evenOddSplit { print(" |", terminator: "") }
-  if i > 0 { print(" ", terminator: "") }
-  print(evenOdd[i], terminator: "")
-}
-print("")
-*/
-
 extension RangeReplaceableCollection {
   public final func myJoin<S : Sequence where S.Iterator.Element == Self>(
     _ elements: S