Eliminate most remaining uses of _convertNSFooToFoo and _convertFooToNSFoo.

Generalized bridging has fully subsumed most of these. NSError is
still special, and _convertStringToNSString remains for the the
runtime's implementation of SwiftObject's -description method.

test/1_stdlib/NewArray.swift.gyb

@@ -226,7 +226,7 @@ func nsArrayOfStrings() -> Array<NSString> {
 
   return src.withUnsafeBufferPointer {
     let ns =  NSArray(objects: UnsafePointer($0.baseAddress), count: $0.count)
-    return _convertNSArrayToArray(ns)
+    return ns as! [NSString]
   }
 }