Remove gravatar_toggle setting

After some thought in #334609: Users should still be able to upload a user picture and override Gravatar, I don't find any reason to use the gravatar_toggle variable. All it is responsible for it disabling all the functionality of the Gravatar module. A user can accomplish the same thing by simply disabling the module in admin/build/modules. Since we're not a huge module where large chunks can be disabled, it is assumed if a user enables this module, they're going to get all the functionality. By using the gravatar_toggle variable, we're unnecessarily increasing complexity and code. It will be easier in the long run to remove it.