refactor: Remove redundant conditional

Summary:
This condition fMaster is already guaranteed to be true by the outer if.

Backport of Core PR17971

Test Plan:
Trivial change. I just made sure I did not introduce any typo by checking that it compiles and tests still run succesfully.
ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7601