Delete unused Atomic32 overloads of binary +/- operators.

Bug: None
Change-Id: I731513aabb45f006112c150ea0e213761cb0716e
Reviewed-on: https://webrtc-review.googlesource.com/2680
Reviewed-by: Henrik Grunell <henrikg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19924}

system_wrappers/include/atomic32.h

@@ -46,11 +46,6 @@ class Atomic32 {
   }
 
  private:
-  // Disable the + and - operator since it's unclear what these operations
-  // should do.
-  Atomic32 operator+(const Atomic32& other);
-  Atomic32 operator-(const Atomic32& other);
-
   // Checks if |_value| is 32bit aligned.
   inline bool Is32bitAligned() const {
     return (reinterpret_cast<ptrdiff_t>(&value_) & 3) == 0;