remove old bootstrap relevant code

Summary:

• only load blockfiles when we have paths
• add release notes for modified bootstrap functionality
• amend documentation on ThreadImport

This is a backport of Core PR17044

Note that the test related to this PR is already backported (D6512)

Test Plan: ninja && ./test/functional/test_runner.py feature_loadblock

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8253