api!: Remove unused info_only option when loading a chatlist (#8171)

Remove unused info_only option from `get_chat_msgs_ex()`.

This option was meant to show an "audit log" of a group, i.e. only the
info messages. This feature was removed again from Desktop, but the
option still lingered around in Core.

This also adds a doc comment to the JsonRPC functions, because I
wanted to note somewhere that the parameter is deprecated, and I needed
some place to put this note.

deltachat-ffi/deltachat.h

@@ -1387,7 +1387,6 @@ dc_msg_t*       dc_get_draft                 (dc_context_t* context, uint32_t ch
 
 
 #define         DC_GCM_ADDDAYMARKER          0x01
-#define         DC_GCM_INFO_ONLY             0x02
 
 
 /**
@@ -1408,7 +1407,6 @@ dc_msg_t*       dc_get_draft                 (dc_context_t* context, uint32_t ch
  * @param flags If set to DC_GCM_ADDDAYMARKER, the marker DC_MSG_ID_DAYMARKER will
  *     be added before each day (regarding the local timezone). Set this to 0 if you do not want this behaviour.
  *     The day marker timestamp is the midnight one for the corresponding (following) day in the local timezone.
- *     If set to DC_GCM_INFO_ONLY, only system messages will be returned, can be combined with DC_GCM_ADDDAYMARKER.
  * @param marker1before Deprecated, set this to 0.
  * @return Array of message IDs, must be dc_array_unref()'d when no longer used.
  */