Unify name of Czech language with CLDR

While MediaWiki core used adverbial form for the Czech language (cs)
in its Names.php, CLDR uses a noun, which is confusing when the
CLDR extension is used, as both variants can be seen in different
places, seemingly randomly.

Therefore, unified to the CLDR variant. See also
http://lists.wikimedia.org/pipermail/mediawiki-i18n/2013-October/000763.html

Change-Id: I2527373c36b2b93f5e11f2d8713f3f81125d1a90