Speed up PO compiling when there are a lot of entries to remove

polib implements deserialized po files as subclasses of `list`. Thus,
hen you want to remove an entry, you can either use `.remove` or `del`.
`.remove` (the one we were using so far) does a serial search in the po
file until it finds the entry to remove. If the po file is big and you
are doing lots of removals, this can become unbearable.

This implementation instead uses `del`. While iterating through the po
file, we add the indexes of the entries we want to remove in a list.
After the iteration is finished, we go through that list in reverse
order and use `del` to remove the entries. Since `del` does not do a
serial search, it is noticeably faster.

openformats/formats/po.py

@@ -111,7 +111,6 @@ def _get_string_data(self, entry):
             'flags': ', '.join(entry.flags),
             'occurrences': occurrences if occurrences else None,
             'developer_comment': entry_comments,
-            'context': entry.msgctxt if entry.msgctxt else ""
         }
 
         string = self._get_string(entry, pluralized)
@@ -270,7 +269,8 @@ def compile(self, template, stringset, **kwargs):
         next_string = next(stringset, None)
 
         po = polib.pofile(template)
-        for entry in list(po):
+        indexes_to_remove = []
+        for i, entry in enumerate(po):
             if next_string is not None:
                 is_plural = True if entry.msgid_plural.strip() else False
                 if is_plural:
@@ -280,7 +280,8 @@ def compile(self, template, stringset, **kwargs):
                 if compiled:
                     next_string = next(stringset, None)
                     continue
-            po.remove(entry)
+            indexes_to_remove.append(i)
+        self._smart_remove(po, indexes_to_remove)
         return unicode(po)
 
     def _compile_entry(self, entry, next_string):
@@ -309,6 +310,10 @@ def _compile_plural_entry(self, entry, next_string):
             return True
         return False
 
+    def _smart_remove(self, po, indexes_to_remove):
+        for i in reversed(indexes_to_remove):
+            del po[i]
+
     @staticmethod
     def _format_occurrences(occurrences):
         """Format the occurrences and return them.