init: Remove boost from ThreadImport

Summary:

Can be tested by calling -reindex or -loadblock and then pressing CTRL+C.

Should print something like:

...
2020-04-27T19:34:31Z [loadblk] Reindexing block file blk00005.dat...
^C2020-04-27T19:34:32Z [loadblk] Shutdown requested. Exit ThreadImport
2020-04-27T19:34:32Z [qt-init] Interrupting HTTP server
...

This is a backport of Core PR18786

Test Plan:
ninja all check-all

Run src/bitcoind -reindex, stop the programs with Ctrl+C (and painfully reindex the whole chain)

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9072