Drop RandomAccessCollection conformance for String.UTF16View

This is consistent with what we've done in the foundation overlay for
OSX.
diff --git a/Foundation/ExtraStringAPIs.swift b/Foundation/ExtraStringAPIs.swift
index 2c737c6..2f5d4f1 100644
--- a/Foundation/ExtraStringAPIs.swift
+++ b/Foundation/ExtraStringAPIs.swift
@@ -29,6 +29,4 @@
     }
 }
 
-extension String.UTF16View : RandomAccessCollection {}
-extension String.UTF16View.Indices : RandomAccessCollection {}