fix(abstract-eth): remove unreachable _counter undefined guard

_counter is typed as `number` and initialized to 0 in the constructor,
making the `_counter === undefined` check in validateBaseTransactionFields
permanently unreachable dead code. Remove it to eliminate a false sense
of safety flagged in the security audit.

Ticket: CGD-576

Author: pritam-gembali

modules/abstract-eth/src/lib/transactionBuilder.ts

@@ -386,9 +386,6 @@ export abstract class TransactionBuilder extends BaseTransactionBuilder {
     if (this._common === undefined) {
       throw new BuildTransactionError('Invalid transaction: network common');
     }
-    if (this._counter === undefined) {
-      throw new BuildTransactionError('Invalid transaction: missing address counter');
-    }
   }
 
   /** @inheritdoc */