Remove SYNC_TRANSACTION_NOT_IN_BLOCK magic number.

Summary: Backport core's PR10186

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: matiu, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1057