Use real value when calculating OutputGroup value

Summary:
OutputGroup::m_value is the true value, not the effective value,
so use the true values of the outputs, not their effective values.

This is a backport of core#17458 [1/2]
https://github.com/bitcoin/bitcoin/pull/17458/commits/9adc2f80fc14f11ee2b1f989ee7be71b58481e6f#

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10088