refactor: Simplify set/get field call to use existing variable (#6534)

The `setFieldU32` call is currently used to set the credential's expiry using a `getFieldU32` call to obtain the expiration time. However, that value was already obtained previously and can thus be reused.

src/libxrpl/tx/transactors/credentials/CredentialCreate.cpp

@@ -107,7 +107,7 @@ CredentialCreate::doApply()
             return tecEXPIRED;
         }
 
-        sleCred->setFieldU32(sfExpiration, ctx_.tx.getFieldU32(sfExpiration));
+        sleCred->setFieldU32(sfExpiration, *optExp);
     }
 
     auto const sleIssuer = view().peek(keylet::account(account_));