protobuf-nano: do not use preexisting IoUtils internal class #3450

This reverts commit 671783f9

The dependency on core caused some problems with
Proguard. There are android builds that should include
protobuf-* but expect the rest of gRPC to be bundled with the
runtime environment. In that case, when Proguard inspects the
output, it will find a reference to IoUtil but fail to find the
class itself. It makes the builds easier to just avoid this
dependency.