multiprocessing forkserver main contains dead main_path= handling code

[gpshead]

Bug report

Bug description:

The multiprocessing module's forkserver start method multiprocessing.forkserver.main has two arguments that are values passed from the parent process to the forkserver when first launching it. main_path= and sys_path=. Via code inspection while fixing and testing #126538:

There is no possible way for main_path= to be set on the multiprocessing's forkserver.main() call since 2013's 9a76735 for #64145 as shipped in 3.4 (which also introduced the forkserver feature).

the forkserver.main(...) call is constructed in Lib/multiprocessing/forkserver.py getting its two possible allowed named args from spawn.get_preparation_data() in Lib/multiprocessing/spawn.py which was changed to set "init_main_from_name" in the kwargs dict it returns instead of "main_path" in the above change. Effectively making the main_path= handling code dead in forkserver.main.

We should either remove the dead code, or determine if it was intended to support something that has been silently broken when using the forkserver start method for the past 11 years, and if so, add an explicit test for that and adapt it to use "init_main_from_name" as the spawn code does.

CPython versions tested on:

3.12, 3.13, 3.14, CPython main branch

Operating systems tested on:

Linux