core -> ABC in extract_strings_qt.py

Summary:
make translate deletes nearly the whole src/qt/bitcoinstrings.cpp file because it's
looking for bitcoin-core and not bitcoin-abc.

Apparently we've never regenerated this file ever?

Test Plan:

../configure
make -j8
cd src
make translate
arc lint

Not committed with this patch, but there should be some sane output in bitcoinstrings.cpp

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5790