Move plural rules of Samogitian(sgs) to plurals-mediawiki.xml

* CLDR does not define plural rules for sgs.
* Port the plural rules present in LanguageSgs.php to CLDR plural
  definition syntax
* Remove LanguageSgs.php
* Update the tests, reorder/rename the plural form names

Change-Id: I44658402d69a6805cdfd189fe780eadee94056c7
Merge "Move plural rules of Samogitian(sgs) to plurals-mediawiki.xml"