libbinder: eliminate Binder.cpp dependency on IShellCallback

Binder.cpp uses IShellCallback::asInterface to convert a Binder
to an IShellCallback, but the return value is not currently used.
This comments out that cast to eliminate the dependency on
IShellCallback, so we can build a minimal libbinder without it.

Bug: 224644083
Test: atest binderRpcTest
Change-Id: I209b1d2fd32cd9538c27aa263a6bf0a7e479fab1