Use -Werror in system/libhwbinder

* Remove unused local variables.
* Suppress warning on unused private data member.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I6295395727ce671a0c40599aa56d97bb21a51778
diff --git a/IPCThreadState.cpp b/IPCThreadState.cpp
index 212bffa..428866a 100644
--- a/IPCThreadState.cpp
+++ b/IPCThreadState.cpp
@@ -710,6 +710,9 @@
     clearCaller();
     mIn.setDataCapacity(256);
     mOut.setDataCapacity(256);
+
+    // TODO(b/67742352): remove this variable from the class
+    (void)mMyThreadId;
 }
 
 IPCThreadState::~IPCThreadState()