cpufreq: amd-pstate: remove unused variable nominal_freq

cpufreq: amd-pstate: remove unused variable nominal_freq

removed the unused variable `nominal_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.

Closes: https://lore.kernel.org/oe-kbuild-all/202405080431.BPU6Yg9s-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Acked-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/b7ef41557f71d40d098393ddb27f0fe1f23648ae.1718811234.git.perry.yuan@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>