libutils: Cleanup unused class declaration

This commit removes unused class declaration for SharedBuffer and
TextOutput.  SharedBuffer has become internal implementation details
since 282efae9c.  TextOutput usages have been removed since 9eb2a3b1.

Test: AOSP and master build w/o problems
Change-Id: I1871c4919a46f1ea8f41fb7eb79b4dc800b6f6f4
2 files changed