Remove ancient code to import keys from ancient plugins

There were a couple specific DRM removal plugins before the DeDRM plugin
was created. These are obsolete since a long time, there's no need to
still have the code to import their config.

If people are still using these ancient plugins, they'll have to update
to an older version of DeDRM first, and then update to the current one.

DeDRM_plugin/__init__.py

@@ -102,8 +102,13 @@ class DeDRMError(Exception):
     pass
 
 from calibre.customize import FileTypePlugin
-from calibre.constants import iswindows, isosx
-from calibre.gui2 import is_ok_to_use_qt
+
+try:
+    from calibre.constants import iswindows, isosx
+except:
+    iswindows = sys.platform.startswith('win')
+    isosx = sys.platform.startswith('darwin')
+
 from calibre.utils.config import config_dir
 
 
@@ -197,10 +202,6 @@ class DeDRM(FileTypePlugin):
                         traceback.print_exc()
                         pass
 
-                # convert old preferences, if necessary.
-                from calibre_plugins.dedrm.prefs import convertprefs
-                convertprefs()
-
                 # mark that this version has been initialized
                 os.mkdir(self.verdir)
         except Exception as e: