Don't form a bound method when calling a C++ operator. (#7345)

Because we now support calling a function with a `self` parameter
directly, we can unconditionally call `operator$(lhs, rhs)` rather than
calling `lhs.operator$(rhs)` if the selected operator function happens
to be a member function.

This makes the logic a bit simpler and the SemIR a bit smaller.

We can't do the same for Carbon operators, unfortunately, as we use the
member access to trigger impl lookup.

toolchain/check/cpp/operators.cpp

@@ -738,11 +738,6 @@ static auto GetAsCppFunctionDecl(Context& context, SemIR::InstId inst_id)
                     : nullptr;
 }
 
-auto IsCppOperatorMethod(Context& context, SemIR::InstId inst_id) -> bool {
-  auto* function_decl = GetAsCppFunctionDecl(context, inst_id);
-  return function_decl && IsCppOperatorMethodDecl(function_decl);
-}
-
 auto IsCppConstructorOrNonMethod(Context& context, SemIR::InstId inst_id)
     -> bool {
   auto* function_decl = GetAsCppFunctionDecl(context, inst_id);