samples: echo_server: Clean up frdm_k64f config
samples: echo_server: Clean up frdm_k64f config

Remove CONFIG_NET_L2_ETHERNET, as it's automatically set by the
board. This effectively makes prj_frdm_k64f.conf fairly portable,
e.g. the same config can be used for frdm_kw41z whcih uses 802.15.4
connectivity instead.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

finikorg/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,aceofall/zephyr-iotos,kraj/zephyr,zephyriot/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,fbsder/zephyr,zephyriot/zephyr,zephyriot/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,fbsder/zephyr,fbsder/zephyr,explora26/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,kraj/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,mbolivar/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,zephyriot/zephyr,Vudentz/zephyr,punitvara/zephyr