refactor: Remove unused credentials signature hash prefix (#6186)

This change removes the unused credentials signature hash prefix from `HashPrefix.h`.

include/xrpl/protocol/HashPrefix.h

@@ -67,9 +67,6 @@ enum class HashPrefix : std::uint32_t {
     /** Payment Channel Claim */
     paymentChannelClaim = detail::make_hash_prefix('C', 'L', 'M'),
 
-    /** Credentials signature */
-    credential = detail::make_hash_prefix('C', 'R', 'D'),
-
     /** Batch */
     batch = detail::make_hash_prefix('B', 'C', 'H'),
 };