Fix invalid comment in WebSettings.

The get/set for the pluginPath is leftover from the Gear's plugin
and is no longer useful now that plugins are loaded from their
own APK using the package manager. These methods were modified to
be no-ops in froyo, but the documentation was never completed.

Change-Id: I20f6593445d877a3dae9b9f51d5198a3509255d6
http://b/2811032