[messagefacility] Remove obsolete Extensions module

As queried on Redmine, the Extensions module of messagefacility
is vestigial to be replaced eventually by a PluginFactory system:

https://cdcvs.fnal.gov/redmine/issues/7110

Remove Extensions subdirectory, and no longer recurse build into this
directory. Remove dynamic loading of MF_ExtModules library in
MessageLoggerScribe.

TODO: Review use of ELdestinationFactory with Extensions in
MessageLoggerScribe::configure_ordinary_destinations()