Avoid unused call to GuessVerificationProgress in NotifyHeaderTip

Summary:

GuessVerificationProgress for a header (not a block) is always 0 because the number of txs in the block can not be determined from the header alone. Anyway, this result was never used, so we can optimize this call by hardcoding 0.

This is the next commit in a series of changes toward removing nChainTx (see #14863, #13875)

This is a backport of Core PR17250

Test Plan: ninja && ninja check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8201