XMLRPC: Switch the file deletion alias to be deleteFile to more closely make uploadFile.

Fixes #5310 props SergeyBiryukov.

Built from https://develop.svn.wordpress.org/trunk@29255


git-svn-id: http://core.svn.wordpress.org/trunk@29038 1a063a9b-81f0-0310-95a4-ce76da25c4cd