debian/upstream/metadata: Drop obsolete Homepage field

debian/upstream/metadata

 Name: poppler
-Homepage: http://poppler.freedesktop.org/
 Bug-Database: https://bugs.freedesktop.org/buglist.cgi?bug_status=__open__&content=&product=poppler&query_format=specific&order=bug_id%20DESC&query_based_on=
 Bug-Submit: https://bugs.freedesktop.org/enter_bug.cgi?product=poppler
 Repository: git://git.freedesktop.org/git/poppler/poppler