samples: net: sockets: echo_service: Use ZVFS default settings

This sample works without increasing ZVFS_OPEN_MAX and ZVFS_POLL_MAX to
some arbitrary number. This also removes a duplicate entry for
ZVFS_OPEN_MAX.

ZVFS_OPEN_MAX already defaults to a reasonably large value of 16 if
POSIX_API is enabled, which is the case for the echo_service sample.

Signed-off-by: Marc Lasch <marc.lasch@husqvarnagroup.com>