message: Remove the unused `ApiError::Validation` variant
diff: @@ -338,12 +338,6 @@ pub enum ApiError {
  /// assert_eq!(ApiError::from(38), ApiError::MissingSystemContractHash);
  /// ```
  MissingSystemContractHash,
  - /// Object was deserialized but it didn't pass validation.
  - /// ```
  - /// # use casper_types::ApiError;
  - /// assert_eq!(ApiError::from(39), ApiError::Validation);
  - /// ```
  - Validation,
  /// Error specific to Auction contract. See
  /// [casper_types::system::auction::Error](crate::system::auction::Error).
  /// ```
  @@ -531,7 +525,6 @@ impl From<ApiError> for u32 {
  ApiError::DictionaryItemKeyExceedsLength => 36,
  ApiError::InvalidDictionaryItemKey => 37,
  ApiError::MissingSystemContractHash => 38,
  - ApiError::Validation => 39,
  ApiError::AuctionError(value) => AUCTION_ERROR_OFFSET + u32::from(value),
  ApiError::ContractHeader(value) => HEADER_ERROR_OFFSET + u32::from(value),
  ApiError::Mint(value) => MINT_ERROR_OFFSET + u32::from(value),
  @@ -582,7 +575,6 @@ impl From<u32> for ApiError {
  36 => ApiError::DictionaryItemKeyExceedsLength,
  37 => ApiError::InvalidDictionaryItemKey,
  38 => ApiError::MissingSystemContractHash,
  - 39 => ApiError::Validation,
  USER_ERROR_MIN..=USER_ERROR_MAX => ApiError::User(value as u16),
  HP_ERROR_MIN..=HP_ERROR_MAX => ApiError::HandlePayment(value as u8),
  MINT_ERROR_MIN..=MINT_ERROR_MAX => ApiError::Mint(value as u8),
  @@ -633,7 +625,6 @@ impl Debug for ApiError {
  ApiError::HostBufferEmpty => write!(f, "ApiError::HostBufferEmpty")?,
  ApiError::HostBufferFull => write!(f, "ApiError::HostBufferFull")?,
  ApiError::AllocLayout => write!(f, "ApiError::AllocLayout")?,
  - ApiError::Validation => write!(f, "ApiError::Validation")?,
  ApiError::DictionaryItemKeyExceedsLength => {
  write!(f, "ApiError::DictionaryItemKeyTooLarge")?
  }
  