samples: net: zperf: Remove unnecessary main
samples: net: zperf: Remove unnecessary main

main function is not necessary.
And without main here, it is possible to import these
samples zperf files in another project.

Signed-off-by: Bub Wei <c983eb7a479178407bca7dfdf005497ee48ddf62@unisoc.com>

GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr