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: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/cpu-freq/index.txt

@@ -32,8 +32,6 @@ cpufreq-stats.txt -	General description of sysfs cpufreq stats.
 
 index.txt	-	File index, Mailing list and Links (this document)
 
-intel-pstate.txt -	Intel pstate cpufreq driver specific file.
-
 pcc-cpufreq.txt -	PCC cpufreq driver specific file.