Remove file IPC at vrtsim startup

[sgarg00]

Originally reported by Bartosz Podrygajlo on 2026-05-20 — GitLab #1095

vrtsim uses two IPC mechanisms:

1. shm_mem - this is the main method of exchanging IQ samples
2. file IPC - this is used to share a descriptor of the shm_mem method from server to client at startup

The file IPC is problematic because if the server process crashes, the file stays on disk, possibly interfering with future runs. We could either:

1. Add information from the file descriptor to clinet config. this would complicate the client config.
2. Move from file IPC to some ephemeral IPC like unix domain sockets. This complicates server code.