QDoc: remove sideeffect from \meta command that causes change in html output

The meta command has one special case that impacts html output as a side effect in addition to its proper responsibility (example tagging in the manifest), for generating an entry for maintainers in class level docs. This has not seen use in Qt 6, Qt 5, or even Qt 4.8 (have not checked further back in time than that). As it is unused, and has been so for more than a decade, remove it.