killing zh_CN.GB2321

fixes https://github.com/owncloud/core/issues/3431 

as described in https://github.com/owncloud/core/issues/3431 zh_CN and zh_CN.GB2312 only differ in the used character set.
Depending on the used browser either one or the other is used which will result in a strange user experience especially because GB2312 has a lower completion percentage

@karlitschek @butonic @ringmaster @bantu @jancborchardt THX