Remove base::OnceCallback<>::Equals

With the current Equals() semantics (BindStateBase pointer comparison),
this should always return false as OnceCallback is a move only type.

BUG=937566

Change-Id: I5477a51721f9c8bf1fa95636263b2095b9588452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497636
Auto-Submit: Robert Liao <robliao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638239}

base/callback.h

@@ -79,8 +79,6 @@ class OnceCallback<R(Args...)> : public internal::CallbackBase {
     return *this;
   }
 
-  bool Equals(const OnceCallback& other) const { return EqualsInternal(other); }
-
   R Run(Args... args) const & {
     static_assert(!sizeof(*this),
                   "OnceCallback::Run() may only be invoked on a non-const "