logging: z_vrfy_log_filter_set: remove extra check

Variable "level" in function z_vrfy_log_filter_set() has type unsigned.
But it is been checked if "level >=LOG_LEVEL_NONE" and
LOG_LEVEL_NONE is 0. It means check if unsigned is ">= 0" in Z_OOPS().
That is logically wrong, because unsigned is ">=0" by default.
Remove that check, to avoid static analysis tool raise
violation

Found as a coding guideline violation (MISRA R14.3) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>