libbinder: remove unused FdTrigger::readFd

Every LOC is a burden on the mind - forgot to remove this when I stopped
using it before.

Bug: N/A
Test: N/A
Change-Id: I0035be2d339d30e0ad5d7fd879232b58bbf4fc3c
diff --git a/libs/binder/include/binder/RpcSession.h b/libs/binder/include/binder/RpcSession.h
index a6bc1a9..59613c5 100644
--- a/libs/binder/include/binder/RpcSession.h
+++ b/libs/binder/include/binder/RpcSession.h
@@ -129,11 +129,6 @@
         static std::unique_ptr<FdTrigger> make();
 
         /**
-         * poll() on this fd for POLLHUP to get notification when trigger is called
-         */
-        base::borrowed_fd readFd() const { return mRead; }
-
-        /**
          * Close the write end of the pipe so that the read end receives POLLHUP.
          */
         void trigger();