[NFC][libc++] Removes sliceExpr friend.

The class is never defined. This was discovered while validating modules
in libc++.

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D148357

diff --git a/libcxx/include/valarray b/libcxx/include/valarray
index c5ed152..76ae4a3 100644 (file)
--- a/libcxx/include/valarray
+++ b/libcxx/include/valarray
@@ -1328,7 +1328,6 @@ private:
         {}
 
     template <class> friend class valarray;
-    template <class> friend class sliceExpr;
 };
 
 template <class _Tp>