Refactor replace_vars: finalize changes

- Removed the $final parameter in favour of using the $omit array to determine what variables not to remove and always removing everything else. Left the filter in place so people can still turn this off if needs be.
- Added some more documentation to the wpseo_register_var_replacement() function
- Prevent registration of replacement variables starting with the WPSEO variable variable prefixes
- Added undoubling of superfluous separators