Revert "fuzz: Add process_messages harness"

Summary:
This reverts commit a9c2983523b2a7378e642255c52cfe502300d0e6.

The CConman initialization in setup_common.cpp is causing a data race when running the tests, and needs to be investigated.
Unbreak TSAN for now.

Test Plan:
With TSAN:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8397