Remove the duplicated RFC7421_PREFIX_LENGTH constants.

The RFC7421_PREFIX_LENGTH has been moved to NetworkStackConstants which
can be shared with modules, remove the duplicated one.

Bug: 290885904
Test: TH
Change-Id: Iac4c33bf442bf7fa7530e4f5d16b0b8db4a3211f
diff --git a/services/net/java/android/net/util/NetworkConstants.java b/services/net/java/android/net/util/NetworkConstants.java
index ea5ce65..49962ea 100644
--- a/services/net/java/android/net/util/NetworkConstants.java
+++ b/services/net/java/android/net/util/NetworkConstants.java
@@ -52,7 +52,6 @@
     public static final int IPV6_ADDR_BITS = 128;
     public static final int IPV6_ADDR_LEN = 16;
     public static final int IPV6_MIN_MTU = 1280;
-    public static final int RFC7421_PREFIX_LENGTH = 64;
 
     /**
      * ICMP common (v4/v6) constants.