With #1549 fixed, there will be no remaining supported use-cases where ptrace-mode works better than preload-mode. We should remove ptrace mode and correspondingly simplify the code.

Since --interpose-method is in the experimental section, this does not require a major version bump.

Let's start by removing the "ptrace" option for the --interpose-method flag, making "preload" currently the only option. Likewise we can remove thread_ptrace.c etc.

A potential follow-up is to remove --interpose-method and support for "subclassing" Thread in general. At the least that should be a distinct separate step, though. We might also consider keeping them around for future experimentation with other interposition mechanisms.

Potentially blocking issues:

• snowflake simulation broken #1549
• shadow-preload tests hang in coverage configuration #867
• Delivery of "synchronous" signals in preload mode (SIGSEGV, SIGILL) #2091