Switch to pullbacks only taking a single input

JuliaDiff/ChainRulesCore.jl#152

src/rulesets/Base/base.jl

@@ -77,14 +77,3 @@ function rrule(::typeof(identity), x)
     end
     return (x, identity_pullback)
 end
-
-function rrule(::typeof(identity), x::Tuple)
-    # `identity(::Tuple)` returns multiple outputs;because that is how we think of
-    # returning a tuple, so its pullback needs to accept multiple inputs.
-    # `identity(::Tuple)` has one input, so its pullback should return 1 matching output
-    # see https://github.com/JuliaDiff/ChainRulesCore.jl/issues/152
-    function identity_pullback(ȳs...)
-        return (NO_FIELDS, Composite{typeof(x)}(ȳs...))
-    end
-    return x, identity_pullback
-end