[stdlib] Remove String.compare()

This was added for benchmarking purposes, without the realization that
it conflicts with the NSString API we adopt when importing Foundation.
http://oleb.net/blog/2014/07/swift-strings/#comparing-strings describes
the problem.

Fixes <rdar://problem/17800504>

Swift SVN r20517

stdlib/core/StringUnicodeScalarView.swift

@@ -222,9 +222,6 @@ extension String {
     self = String(view._core)
   }
 
-  public func compare(other : String) -> Int {
-    return(UnicodeScalarView(_core).compare(UnicodeScalarView(other._core)))
-  }
   public var unicodeScalars : UnicodeScalarView {
     return UnicodeScalarView(_core)
   }