Remove WillDecodeNull layering violation. (#27491)

This should have been done differently to start with, and the only consumer is
gone anyway.

Fixes https://github.com/project-chip/connectedhomeip/issues/23057
diff --git a/src/app/AttributeAccessInterface.h b/src/app/AttributeAccessInterface.h
index 2b16392..8b4ec3a 100644
--- a/src/app/AttributeAccessInterface.h
+++ b/src/app/AttributeAccessInterface.h
@@ -370,11 +370,6 @@
         return CHIP_NO_ERROR;
     }
 
-    /**
-     * Is the next element of the stream NULL.
-     */
-    bool WillDecodeNull() const { return mReader.GetType() == TLV::kTLVType_Null; }
-
     bool TriedDecode() const { return mTriedDecode; }
 
     /**