Simplify the logic to decide whether to show "variant" and "noconvertlink" on Special...

Simplify the logic to decide whether to show "variant" and "noconvertlink" on Special:Preferences

- Group common checks; $wgDisableLangConversion and count( $variants ) only be need to be
  executed once to decide to show or not both fields
- Move the check for existence of multiple variant directly after calling getVariants(),
  so there is no need to run unnecessary code when a language doesn't have variants
- Move the declaration of $variantArray to where it will actually be used
- fix comment style

Change-Id: I2d621424904d0210336845cd82f96bb68a022514