iot-vsocket: Remove redundant `socket_startup.c`

The socket creation is part of
`Middleware/ARM/IoT_VSocket-lib/AVH/interface/vsocket/iot_socket.c` and
hence `socket_startup.c` is redundant. In addition, `socket_startup()`
always returns 0.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>