Revert r45810 "API: (bug 17027) Allow all configuration variables in $wgAPIReadableConfigVars to be read through meta=siteinfo&siprop=configvars. I tried my best to only make useful stuff readable and skip stuff that's sensitive (like $wgDBpassword), available already (like $wgExtraNamespaces) or useless (like $wgUpdateRowsPerJob)."
This is way too tightly coupled to internal implementation details. Any and all of those things could change dramatically; they're not appropriate for an external API.