Revert r45387 "Add special case handling of the XHTML character entity "'" to...

Revert r45387 "Add special case handling of the XHTML character entity "'" to normalizeEntity() and decodeEntity(). This should resolve the remainder of bug 14365.
It might seem cleaner to just add the appropriate entry to $wgHtmlEntityAliases, but this would break decodeEntity() as currently written. Explicitly note this in the comments."

This doesn't make any sense. Is there any reason not to just add apos to $wgHtmlEntities if it's valid XHTML?