Settings: Remove redundancy in description

* All these settings are under a 'Volume buttons' header.
  Avoiding duplication of 'Volume button' makes it easier for
  translators

Change-Id: I51eb2de69c2f038f13d95a8e97bde1ecf83df913
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_packages_apps_Settings into cm-11.0