Remove dumpers from the public API.

All those dumper were not used by any of our other projects.
They are only used internally, either by `Library::writeToFile` or the
server.

include/meson.build

@@ -4,10 +4,6 @@ headers = [
   'common.h',
   'library.h',
   'manager.h',
-  'libxml_dumper.h',
-  'opds_dumper.h',
-  'library_dumper.h',
-  'html_dumper.h',
   'downloader.h',
   'search_renderer.h',
   'server.h',