cpufreq: intel_pstate: Drop INTEL_PSTATE_HWP_SAMPLING_INTERVAL

After commit 62611cb9 (intel_pstate: delete scheduler hook in HWP
mode) the INTEL_PSTATE_HWP_SAMPLING_INTERVAL is not used anywhere in
the code, so drop it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>