Remove unused DB fields

This field was added but never used

This does not remove the columns from the DB, but doesnt add them
in new migrations.

gajim/common/check_paths.py

@@ -75,7 +75,6 @@ def create_log_db():
                     subject TEXT,
                     additional_data TEXT DEFAULT '{}',
                     stanza_id TEXT,
-                    mam_id TEXT,
                     encryption TEXT,
                     encryption_state TEXT,
                     marker INTEGER

gajim/common/optparser.py

@@ -878,7 +878,6 @@ class OptionsParser:
     def update_config_to_016103(self):
         self.call_sql(logger.LOG_DB_PATH,
             '''ALTER TABLE logs ADD COLUMN 'stanza_id' TEXT;
-            ALTER TABLE logs ADD COLUMN 'mam_id' TEXT;
             ALTER TABLE logs ADD COLUMN 'encryption' TEXT;
             ALTER TABLE logs ADD COLUMN 'encryption_state' TEXT;
             ALTER TABLE logs ADD COLUMN 'marker' INTEGER;