Remove unused parameter validFeeEstimates from CTxMempool::addUnchecked

Summary:
Remove unused parameter validFeeEstimates from CTxMempool::addUnchecked, and the
associated function for calculating this flag (IsCurrentForFeeEstimation).

The code which made use of this parameter was removed with:

• Delete block policy estimator. https://reviews.bitcoinabc.org/D2172

Test Plan:

make VERBOSE=1 check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2784