core: remove thisT() from API

diff --git a/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java b/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
index 7ccfaa7..6d5578b 100644
--- a/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
+++ b/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
@@ -422,4 +422,13 @@
       return DIRECT_ADDRESS_SCHEME;
     }
   }
+
+  /**
+   * Returns the correctly typed version of the builder.
+   */
+  private T thisT() {
+    @SuppressWarnings("unchecked")
+    T thisT = (T) this;
+    return thisT;
+  }
 }