cpufreq: docs: Drop intel-pstate.txt from index.txt

Commit 33fc30b4 (cpufreq: intel_pstate: Document the current
behavior and user interface) dropped the intel-pstate.txt file
from Documentation/cpu-freq/, but it did not update the index.txt
file in there accordingly, so do that now.

Fixes: 33fc30b4

 (cpufreq: intel_pstate: Document the current behavior and user interface)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>