cpufreq: amd-pstate: remove unused variable lowest_nonlinear_freq

cpufreq: amd-pstate: remove unused variable lowest_nonlinear_freq

removed the unused variable `lowest_nonlinear_freq` for build warning.
This variable was defined and assigned a value in the previous code,
but it was not used in the subsequent code.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404271038.em6nJjzy-lkp@intel.com/
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>