Tighten up the extraction I/O

We were writing the main file twice.  No need.

i18n/extract.py

@@ -41,9 +41,6 @@ def main():
     create_dir_if_necessary(LOCALE_DIR)
     source_msgs_dir = CONFIGURATION.source_messages_dir
 
-    generated_files = ['django-partial.po', 'djangojs.po', 'mako.po']
-
-    # Prepare makemessages command.
     makemessages = "django-admin.py makemessages -l en"
     ignores = " ".join('--ignore="{}/*"'.format(d) for d in CONFIGURATION.ignore_dirs)
     if ignores:
@@ -70,10 +67,9 @@ def main():
 
     # Segment the generated files.
     segmented_files = segment_pofiles("en")
-    generated_files.extend(segmented_files)
 
     # Finish each file.
-    for filename in generated_files:
+    for filename in segmented_files:
         LOG.info('Cleaning %s' % filename)
         po = pofile(source_msgs_dir.joinpath(filename))
         # replace default headers with edX headers

i18n/segment.py

@@ -93,13 +93,13 @@ def segment_pofile(filename, segments):
             else:
                 msg_segments.add(filename)
 
+        assert msg_segments
         if len(msg_segments) == 1:
             # This message belongs in this segment.
             segment_file = msg_segments.pop()
             segment_po_files[segment_file].append(msg)
         else:
-            # Either it's in more than one segment, or none, so put it back in
-            # the main file.
+            # It's in more than one segment, so put it back in the main file.
             remaining_po.append(msg)
 
     # Write out the results.
@@ -113,9 +113,6 @@ def segment_pofile(filename, segments):
             pofile.save(out_file)
             files_written.add(out_file)
 
-    LOG.info(writing_msg.format(file=filename, num=len(remaining_po)))
-    remaining_po.save(filename)
-
     return files_written