Remove {Dictionary,Set,UnsafeMutableBufferPointer,UnsafeBufferPointer}.count

These APIs are redundant with APIs that come from protocol extensions.

Swift SVN r28248

validation-test/stdlib/UnicodeTrie.swift.gyb

@@ -119,7 +119,7 @@ class NonContiguousNSString : NSString {
   }
 
   @objc override var length: Int {
-    return _value.count
+    return _value.count()
   }
 
   @objc override func characterAtIndex(index: Int) -> unichar {