Remove classes and functions deprecated in MediaWiki 1.17

Removed classes:
- WikiError (deprecated in 1.17)
- WikiXmlError (deprecated in 1.17)
- WikiErrorMsg (deprecated in 1.17)

Removed methods:
- ApiBase::getValidNamespaces() (deprecated in 1.17)
- ApiMain::setCachePrivate() (deprecated in 1.17)
- ApiMain::setVaryCookie (deprecated in 1.17)
- CategoryViewer::addSubcategory() (deprecated in 1.17)
- WebRequest::getFileSize() (deprecated in 1.17)
- WebRequest::isPathInfoBad() (deprecated in 1.17)

Change-Id: Id715c2391742c136d09f5369f88d67e798842af7
Merge "Remove classes and functions deprecated in MediaWiki 1.17"