Remove custom plurals for Nso and Sl in favour of CLDR

Nso - Northern Sotho
Sl - Slovenian

Plural rules were not changed. They are same in CLDR and MW

Change-Id: I0e0c84352de2de8f58af5a9147ba18b0fe1fb39a
Merge "Remove custom plurals for Nso and Sl in favour of CLDR"