Remove chat_log_get_previous()

We now dont get the log files from the text files via chat_log_get_previous() anymore.
We use the sql backend via log_database_get_previous_chat().

So far it just has the same behaviour like chat_log_get_previous(),
except that in _chatwin_history() we don't pass the sender to
win_print_history() which should be fixed in a commit soon.

And log_database_get_previous_chat() can later easily be expanded to fix
https://github.com/profanity-im/profanity/issues/205.

src/log.h

@@ -82,7 +82,6 @@ void chat_log_pgp_msg_in(ProfMessage *message);
 void chat_log_omemo_msg_in(ProfMessage *message);
 
 void chat_log_close(void);
-GSList* chat_log_get_previous(const gchar *const login, const gchar *const recipient);
 
 void groupchat_log_init(void);