Remove check to see if lock / unlock are already defined

Crosswalk Cordova implements the screen orientation API which conflicts
with the plugin. The plugin is courteous and doesn't add its fucntions
to the screen object if they already exist. Unfortunately, we need a function
which is unimplemented by the Crosswalk orientation API (isLocked), so we have
to overwrite their implementation with ours. This requires removing the courteousness
check.