shell: Remove unnecessary initialization of err
shell: Remove unnecessary initialization of err

All code paths in shell_exec() are guaranteed to set err before
returning it, so doing this (rather curious) initialization is
completely unnecessary.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

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