samples: net: zperf: Remove invalid forever sleep

We are not expecting forever k_sleep in multithreading.
And k_sleep is empty in non-multihreading.
So remove invalid forever sleep to avoid the below issue.

ASSERTION FAIL [duration != (-1)] @ zephyr/kernel/sched.c:807

Please refer to _impl_k_sleep api in zephyr/kernel/sched.c:807

Signed-off-by: Bub Wei <bub.wei@unisoc.com>