Fix for rebase/merge failure

Summary:
Fixes 39fe309bab18c7e00a66e9f97a2cf47f7fc6d30d.
LoadExternalBlockFile(..) takes chain parameters out of
the config object now, so the signature changed
correspondingly.

Test Plan: `src/test/test_bitcoin -t validation_tests```

Reviewers: kyuupichan, freetrader, sickpig, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D192